Mr. Chief Justice Vinson
delivered the opinion of the Court.
These cases present for decision the constitutionality of § 9 (h) of the National Labor Relations Act, as amended by the Labor Management Relations Act, 1947.1 This section, commonly referred to as the non-Communist affidavit provision, reads as follows: “No investigation shall be made by the [National Labor Relations] Board of any question affecting commerce concerning the representation of employees, raised by a labor organization under subsection (c) of this section, no petition under section 9 (e) (1) shall be entertained, and no complaint shall be issued pursuant to a charge made by a labor organization under subsection (b) of section 10, unless there is on file with the Board an affidavit executed contemporaneously or within the preceding twelve-month period by each officer of such labor organization and the officers of any national or international labor organization of which it is an affiliate or [386]*386constituent unit that he is not a member of the Communist Party or affiliated with such party, and that he does not believe in, and is not a member of or supports any organization that believes in or teaches, the overthrow of the United States Government by force or by any illegal or unconstitutional methods. The provisions of section 35 A of the Criminal Code shall be applicable in respect to such affidavits.”
In No. 10, the constitutional issue was raised by a suit to restrain the Board from holding a representation election in a bargaining unit in which appellant union was the employee representative, without permitting its name to appear on the ballot, and, should the election be held, to restrain the Board from announcing the results or certifying the victor, until a hearing was granted to appellant. A hearing had been denied because of the noncompliance with § 9 (h). The complaint alleged that this requirement was unconstitutional. Appellee’s motion to dismiss the complaint was granted by the statutory three-judge court, 79 F. Supp. 563 (1948), with one judge dissenting. Since the constitutional issues were properly raised and substantial, we noted probable jurisdiction.
No. 13 is the outcome of an unfair labor practice complaint filed with the Board by petitioner unions. The Board found that Inland Steel Company had violated the Labor Relations Act in refusing to bargain on the subject of pensions. 77 N. L. R. B. 1 (1948). But the Board postponed the effective date of its order compelling the company to bargain, pending the unions’ compliance with § 9 (h). Both sides appealed: the company urged that the Act had been misinterpreted; the unions contended that § 9 (h) was unconstitutional and therefore an invalid condition of a Board order. When the court below upheld the Board on both counts, 170 F. 2d 247 (1948), with one judge dissenting as to § 9 (h), both sides filed petitions for certiorari. We denied the petition pertain[387]*387ing to the pension issue, 336 U. S. 960 (1949), but granted the petition directed at the affidavit requirement, 335 U. S. 910 (1949), because of the manifest importance of the constitutional issues involved.
I.
The constitutional justification for the National Labor Relations Act was the power of Congress to protect interstate commerce by removing obstructions to the free flow of commerce. National Labor Relations Board v. Jones & Laughlin Steel Corp., 301 U. S. 1 (1937). That Act was designed to remove obstructions caused by strikes and other forms of industrial unrest, which Congress found were attributable to the inequality of bargaining power between unorganized employees and their employers. It did so by strengthening employee groups, by restraining certain employer practices, and by encouraging the processes of collective bargaining.
When the Labor Management Relations Act was passed twelve years later, it was the view of Congress that additional impediments to the free flow of commerce made amendment of the original Act desirable. It was stated in the findings and declaration of policy that:
“Experience has further demonstrated that certain practices by some labor organizations, their officers, and members have the intent or the necessary effect of burdening or obstructing commerce by preventing the free flow of goods in such commerce through strikes and other forms of industrial unrest or through concerted activities which impair the interest of the public in the free flow of such commerce. The elimination of such practices is a necessary condition to the assurance of the rights herein guaranteed.” 2
[388]*388One such obstruction, which it was the purpose of § 9 (h) of the Act to remove, was the so-called “political strike.” Substantial amounts of evidence were presented to various committees of Congress, including the committees immediately concerned with labor legislation, that Communist leaders of labor unions had in the past and would continue in the future to subordinate legitimate trade union objectives to obstructive strikes when dictated by Party leaders, often in support of the policies of a foreign government. And other evidence supports the view that some union leaders who hold to a belief in violent overthrow of the Government for reasons other than loyalty to the Communist Party likewise regard strikes and other forms of direct action designed to serve ultimate revolutionary goals as the primary objectives of labor unions which they control.3 At the committee hearings, the incident most fully developed was a strike at the Milwaukee plant of the Allis-Chalmers Manufacturing Company in 1941, when that plant was producing vital materials for the national defense program. A full hearing was given not only to company officials, but also to leaders of the international and local unions involved. Congress heard testimony that the strike had been called solely in obedience to Party orders for the purpose of starting the “snowballing of strikes” in defense plants.4
No useful purpose would be served by setting out at length the evidence before Congress relating to the prob[389]*389lem of political strikes, nor can we attempt to assess the validity of each item of evidence. It is sufficient to say that Congress had a great mass of material before it which tended to show that Communists and others proscribed by the statute had infiltrated union organizations not to support and further trade union objectives, including the advocacy of change by democratic methods, but to make them a device by which commerce and industry might be disrupted when the dictates of political policy required such action.
II.
The unions contend that the necessary effect of § 9 (h) is to make it impossible for persons who cannot sign the oath to be officers of labor unions. They urge that such a statute violates fundamental rights guaranteed by the First Amendment: the right of union officers to hold what political views they choose and to associate with what political groups they will, and the right of unions to choose their officers without interference from government.5 The Board has argued, on the other hand, that § 9 (h) presents no First Amendment problem because its sole sanction is the withdrawal from noncomplying unions of the “privilege” of using its facilities.
Neither contention states the problem with complete accuracy. It cannot be denied that the practical effect of denial of access to the Board and the denial of a place on the ballot in representation proceedings is not merely to withhold benefits granted by the Government but to impose upon noncomplying unions a number of restrictions which would not exist if the Board had not been [390]*390established.6 The statute does not, however, specifically forbid persons who do not sign the affidavit from holding positions of union leadership nor require their discharge from office. The fact is that § 9 (h) may well make it difficult for unions to remain effective if their officers do not sign the affidavits. How difficult depends upon the circumstances of the industry, the strength of the union and its organizational discipline. We are, therefore, neither free to treat § 9 (h) as if it merely withdraws a privilege gratuitously granted by the Government, nor able to consider it a licensing statute prohibiting those persons who do not sign the affidavit from holding union office. The practicalities of the situation place the proscriptions of § 9 (h) somewhere between those two extremes. The difficult question that emerges is whether, consistently with the First Amendment, Congress, by statute, may exert these pressures upon labor unions to deny positions of leadership to certain persons who are identified by particular beliefs and political affiliations.
III.
There can be no doubt that Congress may, under its constitutional power to regulate commerce among the several States, attempt to prevent political strikes and other kinds of direct action designed to burden and interrupt the free flow of commerce. We think it is clear, in addition, that the remedy provided by § 9 (h) bears rea[391]*391sonable relation to the evil which the statute was designed to reach. Congress could rationally find that the Communist Party is not like other political parties in its utilization of positions of union leadership as means by which to bring about strikes and other obstructions of commerce for purposes of political advantage, and that many persons who believe in overthrow of the Government by force and violence are also likely to resort to such tactics when, as officers, they formulate union policy.
The fact that the statute identifies persons by their political affiliations and beliefs, which are circumstances ordinarily irrelevant to permissible subjects of government action, does not lead to the conclusion that such circumstances are never relevant. In re Summers, 325 U. S. 561 (1945); Hamilton v. Regents, 293 U. S. 245 (1934). We have held that aliens may be barred from certain occupations because of a reasonable relation between that classification and the apprehended evil, Clarke v. Deckebach, 274 U. S. 392 (1927); Pearl Assurance Co. v. Harrington, 313 U. S. 549 (1941), even though the Constitution forbids arbitrary banning of aliens from the pursuit of lawful occupations. Truax v. Raich, 239 U. S. 33 (1915); Takahashi v. Fish and Game Commission, 334 U. S. 410 (1948). Even distinctions based solely on ancestry, which we declared “are by their very nature odious to a free people,” have been upheld under the unusual circumstances of wartime. Hirabayashi v. United States, 320 U. S. 81 (1943).7 If accidents of birth and ancestry under some circumstances justify an inference concerning future conduct, it can hardly be doubted that voluntary affiliations and beliefs justify a similar inference when drawn by the legislature on the basis of its investigations.
[392]*392This principle may be illustrated by reference to statutes denying positions of public importance to groups of persons identified by their business affiliations. One federal statute,8 for example, provides that no partner or employee of a firm primarily engaged in underwriting securities may be a director of a national bank. This Court noted that the statute is directed “to the probability or likelihood, based on the experience of the 1920’s, that a bank director interested in the underwriting business may use his influence in the bank to involve it or its customers in securities which his underwriting house has in its portfolio or has committed itself to take.” Board of Governors v. Agnew, 329 U. S. 441, 447 (1947). It was designed “to remove tempting opportunities from the management and personnel of member banks.” Id. at p. 449. There was no showing, nor was one required, that all employees of underwriting firms would engage in such conduct. Because of their business connections, carrying as they do certain loyalties, interests and disciplines, those persons were thought to pose a continuing threat of participation in the harmful activities described above. Political affiliations of the kind here involved, no less than business affiliations, provide rational ground for the legislative judgment that those persons proscribed by § 9 (h) would be subject to “tempting opportunities” to commit acts deemed harmful to the national economy. In this respect, § 9 (h) is not unlike a host of other statutes which prohibit specified groups of persons from holding positions of power and public interest because, in the legislative judgment, they threaten to abuse the trust that is a necessary concomitant of the power of office.
If no more were involved than possible loss of position, the foregoing would dispose of the case. But the more [393]*393difficult problem here arises because, in drawing lines on the basis of beliefs and political affiliations, though it may be granted that the proscriptions of the statute bear a reasonable relation to the apprehended evil, Congress has undeniably discouraged the lawful exercise of political freedoms as well. Stated otherwise, the problem is this: Communists, we may assume, carry on legitimate political activities. Beliefs are inviolate. Cantwell v. Connecticut, 310 U. S. 296, 303 (1940). Congress might reasonably find, however, that Communists, unlike members of other political parties, and persons who believe in overthrow of the Government by force, unlike persons of other beliefs, represent a continuing danger of disruptive political strikes when they hold positions of union leadership. By exerting pressures on unions to deny office to Communists and others identified therein, § 9 (h) undoubtedly lessens the threat to interstate commerce, but it has the further necessary effect of discouraging the exercise of political rights protected by the First Amendment. Men who hold union offices often have little choice but to renounce Communism or give up their offices. Unions which wish to do so are discouraged from electing Communists to office. To the grave and difficult problem thus presented we must now turn our attention.
IV.
The unions contend that once it is determined that this is a free speech case, the “clear and present danger” test must apply. See Schenck v. United States, 249 U. S. 47 (1919). But they disagree as to how it should be applied. Appellant in No. 10 would require that joining the Communist Party or the expression of belief in overthrow of the Government by force be shown to be a clear and present danger of some substantive evil, since those are the doctrines affected by the statute. Peti[394]*394tioner in No. 13, on the other hand, would require a showing that political strikes, the substantive evil involved, are a clear and present danger to the security of the Nation or threaten widespread industrial unrest.
This confusion suggests that the attempt to apply the term, “clear and present danger,” as a mechanical test in every case touching First Amendment freedoms, without regard to the context of its application, mistakes the form in which an idea was cast for the substance of the idea. The provisions of the Constitution, said Mr. Justice Holmes, “are not mathematical formulas having their essence in their form; they are organic living institutions transplanted from English soil. Their significance is vital not formal; it is to be gathered not simply by taking the words and a dictionary, but by considering their origin and the line of their growth.” Gompers v. United States, 233 U. S. 604, 610 (1914). Still less should this Court’s interpretations of the Constitution be reduced to the status of mathematical formulas. It is the considerations that gave birth to the phrase, “clear and present danger,” not the phrase itself, that are vital in our decision of questions involving liberties protected by the First Amendment.
Although the First Amendment provides that Congress shall make no law abridging the freedom of speech, press or assembly, it has long been established that those freedoms themselves are dependent upon the power of constitutional government to survive. If it is to survive it must have power to protect itself against unlawful conduct and, under some circumstances, against incitements to commit unlawful acts. Freedom of speech thus does not comprehend the right to speak on any subject at any time. The important question that came to this Court immediately after the First World War was not whether, but how far, the First Amendment permits the suppression of speech which advocates conduct inimical [395]*395to the public welfare.9 Some thought speech having a reasonable tendency to lead to such conduct might be punished. Justices Holmes and Brandéis took a different view. They thought that the greater danger to a democracy lies in the suppression of public discussion; that ideas and doctrines thought harmful or dangerous are best fought with words. Only, therefore, when force is very likely to follow an utterance before there is a chance for counter-argument to have effect may that utterance be punished or prevented.10 Thus, “the necessity which is essential to a valid restriction does not exist unless speech would produce, or is intended to produce, a clear and imminent danger of some substantive evil which the State [or Congress] constitutionally may seek to prevent . . . Mr. Justice Brandeis, concurring in Whitney v. California, 274 U. S. 357, 373. By this means they sought to convey the philosophy that, under the First Amendment, the public has a right to every man’s views and every man the right to speak them. Government may cut him off only when his views are no longer merely views but threaten, clearly and imminently, to ripen into conduct against which the public has a right to protect itself.
[396]*396But the question with which we are here faced is not the same one that Justices Holmes and Brandéis found convenient to consider in terms of clear and present danger. Government’s interest here is not in preventing the dissemination of Communist doctrine or the holding of particular beliefs because it is feared that unlawful action will result therefrom if free speech is practiced. Its interest is in protecting the free flow of commerce from what Congress considers to be substantial evils of conduct that are not the products of speech at all. Section 9 (h), in other words, does not interfere with speech because Congress fears the consequences of speech; it regulates harmful conduct which Congress has determined is carried on by persons who may be identified by their political affiliations and beliefs. The Board does not contend that political strikes, the substantive evil at which § 9 (h) is aimed, are the present or impending products of advocacy of the doctrines of Communism or the expression of belief in overthrow of the Government by force. On the contrary, it points out that such strikes are called by persons who, so Congress has found, have the will and power to do so without advocacy or persuasion that seeks acceptance in the competition of the market.11 Speech may be fought with speech. Falsehoods and fallacies must be exposed, not suppressed, unless there is not sufficient time to avert the evil consequences of noxious doctrine by argument and education. That is the command of the First Amendment. But force may and must be met with force. Section 9 (h) is designed to protect the public not against what Communists and others identified therein advocate or believe, but against what Congress has concluded they have done and are likely to do again.
[397]*397The contention of petitioner in No. 13 that this Court must find that political strikes create a clear and present danger to the security of the Nation or of widespread industrial strife in order to sustain § 9 (h) similarly misconceives the purpose that phrase was intended to serve. In that view, not the relative certainty that evil conduct will result from speech in the immediate future, but the extent and gravity of the substantive evil must be measured by the “test” laid down in the Schenck case. But there the Court said that: “The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent.” Schenck v. United States, supra at 52. (Emphasis supplied.)
So far as the Schenck case itself is concerned, imminent danger of any substantive evil that Congress may prevent justifies the restriction of speech. Since that time this Court has decided that however great the likelihood that a substantive evil will result, restrictions on speech and press cannot be sustained unless the evil itself is “substantial” and “relatively serious,” Brandéis, J., concurring in Whitney v. California, supra at 374, 377, or sometimes “extremely serious,” Bridges v. California, 314 U. S. 252, 263 (1941). And it follows therefrom that even harmful conduct cannot justify restrictions upon speech unless substantial interests of society are at stake. But in suggesting that the substantive evil must be serious and substantial, it was never the intention of this Court to lay down an absolutist test measured in terms of danger to the Nation. When the effect of a statute or ordinance upon the exercise of First Amendment freedoms is relatively small and the public interest to be protected is substantial, it is obvious that a rigid test requiring a showing of imminent danger to the security of the Nation is an absurdity. We recently dismissed for want of sub[398]*398stantiality an appeal in which a church group contended that its First Amendment rights were violated by a municipal zoning ordinance preventing the building of churches in certain residential areas. Corporation of the Presiding Bishop of the Church of Jesus Christ of Latter-Day Saints v. Porterville, 338 U. S. 805 (1949). And recent cases in this Court involving contempt by publication likewise have no meaning if imminent danger of national peril is the criterion.12
On the contrary, however, the right of the public to be protected from evils of conduct, even though First Amendment rights of persons or groups are thereby in some manner infringed, has received frequent and consistent recognition by this Court. We have noted that the blaring sound truck invades the privacy of the home and may drown out others who wish to be heard. Kovacs v. Cooper, 336 U. S. 77 (1949). The unauthorized parade through city streets by a religious or political group disrupts traffic and may prevent the discharge of the most essential obligations of local government. Cox v. New Hampshire, 312 U. S. 569, 574 (1941). The exercise of particular First Amendment rights may fly in the face of the public interest in the health of children, Prince v. Massachusetts, 321 U. S. 158 (1944), or of the whole community, Jacobson v. Massachusetts, 197 U. S. 11 (1905), and it may be offensive to the moral standards of the community, Reynolds v. United States, 98 U. S. 145 (1878); Davis v. Beason, 133 U. S. 333 (1890). And Government’s obligation to provide an efficient public service, United Public Workers v. Mitchell, 330 U. S. 75 (1947), and its interest in the character of members of the bar, In re Summers, 325 U. S. 561 (1945), sometimes admit of limitations upon rights set out in the First Amendment. And see Giboney v. Empire Storage Co., [399]*399336 U. S. 490, 499-501 (1949). We have never held that such freedoms are absolute. The reason is plain. As Mr. Chief Justice Hughes put it, “Civil liberties, as guaranteed by the Constitution, imply the existence of an organized society maintaining public order without which liberty itself would be lost in the excesses of unrestrained abuses.” Cox v. New Hampshire, supra at 574.
When particular conduct is regulated in the interest of public order, and the regulation results in an indirect, conditional, partial abridgment of speech, the duty of the courts is to determine which of these two .conflicting interests demands the greater protection under the particular circumstances presented. The high place in which the right to speak, think, and assemble as you will was held by the Framers of the Bill of Rights and is held today by those who value liberty both as a means and an end indicates the solicitude with which we must view any assertion of personal freedoms. We must recognize, moreover, that regulation of “conduct” has all too frequently been employed by public authority as a cloak to hide censorship of unpopular ideas. We have been reminded that “It is not often in this country that we now meet with direct and candid efforts to stop speaking or publication as such. Modern inroads on these rights come from associating the speaking with some other factor which the state may regulate so as to bring the whole within official control.”13
On the other hand, legitimate attempts to protect the public, not from the remote possible effects of noxious ideologies, but from present excesses of direct, active conduct, are not presumptively bad because they interfere with and, in some of its manifestations, restrain the exercise of First Amendment rights. Reynolds v. United States, supra; Prince v. Massachusetts, supra; Cox v. [400]*400New Hampshire, supra; Giboney v. Empire Storage Co., supra. In essence, the problem is one of weighing the probable effects of the statute upon the free exercise of the right of speech and assembly against the congressional determination that political strikes are evils of conduct which cause substantial harm to interstate commerce and that Communists and others identified by § 9 (h) pose continuing threats to that public interest when in positions of union leadership. We must, therefore, undertake the “delicate and difficult task ... to weigh the circumstances and to appraise the substantiality of the reasons advanced in support of the regulation of the free enjoyment of the rights.” Schneider v. State, 308 U. S. 147, 161 (1939).
V.
The “reasons advanced in support of the regulation” are of considerable weight, as even the opponents of § 9 (h) agreed. They are far from being “[m]ere legislative preferences or beliefs respecting matters of public convenience [which] may well support regulation directed at other personal activities, but be insufficient to justify such as diminishes the exercise of rights so vital to the maintenance of democratic institutions.” 14 It should be emphasized that Congress, not the courts, is primarily charged with determination of the need for regulation of activities affecting interstate commerce. This Court must, if such regulation unduly infringes personal freedoms, declare the statute invalid under the First Amendment’s command that the opportunities for free public discussion be maintained. But insofar as the problem is one of drawing inferences concerning the need for regulation of particular forms of conduct from conflicting evidence, this Court is in no position to substitute its judgment as to the necessity or desirability of the stat[401]*401ute for that of Congress. Cf. United Public Workers v. Mitchell, supra at 95, 102. In Bridges v. California, supra, we said that even restrictions on particular kinds of utterances, if enacted by a legislature after appraisal of the need, come to this Court “encased in the armor wrought by prior legislative deliberation.” 314 U. S. at 261. Compare Gitlow v. New York, 268 U. S. 652 (1925). The deference due legislative determination of the need for restriction upon particular forms of conduct has found repeated expression in this Court’s opinions.
When compared with ordinances and regulations dealing with littering of the streets or disturbance of householders by itinerant preachers, the relative significance and complexity of the problem of political strikes and how to deal with their leaders becomes at once apparent. It must be remembered that § 9 (h) is not an isolated statute dealing with a subject divorced from the problems of labor peace generally. It is a part of some very complex machinery set up by the Federal Government for the purpose of encouraging the peaceful settlement of labor disputes. Under the statutory scheme, unions which become collective bargaining representatives for groups of employees often represent not only members of the union but nonunion workers or members of other unions as well. Because of the necessity to have strong unions to bargain on equal terms with strong employers, individual employees are required by law to sacrifice rights which, in some cases, are valuable to them. See J. I. Case Co. v. Labor Board, 321 U. S. 332 (1944). The loss of individual rights for the greater benefit of the group results in a tremendous increase in the power of the representative of the group — the union. But power is never without responsibility. And when authority derives in part from Government’s thumb on the scales, the exercise of that power by private persons becomes closely akin, in some respects, to its exercise by Government itself. [402]*402See Graham v. Brotherhood of Locomotive Firemen, 338 U. S. 232 (1949); Steele v. Louisville & N. R. Co., 323 U. S. 192 (1944); Tunstall v. Brotherhood of Locomotive Firemen, 323 U. S. 210 (1944); Wallace Corp. v. Labor Board, 323 U. S. 248, 255 (1944); Railway Mail Association v. Corsi, 326 U. S. 88, 94 (1945).
We do not suggest that labor unions which utilize the facilities of the National Labor Relations Board become Government agencies or may be regulated as such. But it is plain that when Congress clothes the bargaining representative “with powers comparable to those possessed by a legislative body both to create and restrict the rights of those whom it represents,”15 the public interest in the good faith exercise of that power is very great.
What of the effects of § 9 (h) upon the rights of speech and assembly of those proscribed by its terms? The statute does not prevent or punish by criminal sanctions the making of a speech, the affiliation with any organization, or the holding of any belief. But as we have noted, the fact that no direct restraint or punishment is imposed upon speech or assembly does not determine the free speech question. Under some circumstances, indirect “discouragements” undoubtedly have the same coercive effect upon the exercise of First Amendment rights as imprisonment, fines, injunctions or taxes. A requirement that adherents of particular religious faiths or political parties wear identifying arm-bands, for example, is obviously of this nature.
But we have here no statute which is either frankly aimed at the suppression of dangerous ideas16 nor one [403]*403which, although ostensibly aimed at the regulation of conduct, may actually “be made the instrument of arbitrary suppression of free expression of views.” Hague v. Committee for Industrial Organization, 307 U. S. 496, 516 (1939).17 There are here involved none of the elements of censorship or prohibition of the dissemination of information that were present in the cases mainly relied upon by those attacking the statute.18 The “discouragements” of § 9 (h) proceed, not against the groups or beliefs identified therein, but only against the combination of [404]*404those affiliations or beliefs with occupancy of a position of great power over the economy of the country. Congress has concluded that substantial harm, in the form of direct, positive action, may be expected from that combination. In this legislation, Congress did not restrain the activities of the Communist Party as a political organization; nor did it attempt to stifle beliefs. Compare West Virginia State Board of Education v. Barnette, 319 U. S. 624 (1943).19 Section 9 (h) touches only a relative handful of persons, leaving the great majority of persons of the identified affiliations and beliefs completely free from restraint. And it leaves those few who are affected free to maintain their affiliations and beliefs subject only to possible loss of positions which Congress has concluded are being abused to the injury of the public by members of the described groups.
We have previously had occasion to consider other statutes and regulations in which the interests involved were, in large measure, like those now being considered. In United Public Workers v. Mitchell, supra, we upheld [405]*405a statute which provided that employees of the Federal Government could not participate in partisan political activities, concededly a First Amendment right, if they would retain their positions. The decision was not put upon the ground that government employment is a privilege to be conferred or withheld at will. For it was recognized that Congress may not "enact a regulation providing that no Republican, Jew or Negro shall be appointed to federal office, or that no federal employee shall attend Mass or take any active part in missionary work.” 330 U. S. at 100. But the rational connection between the prohibitions of the statute and its objects, the limited scope of the abridgment of First Amendment rights, and the large public interest in the efficiency of government service, which Congress had found necessitated the statute, led us to the conclusion that the statute may stand consistently with the First Amendment.
Similarly, in In re Summers, supra, we upheld the refusal of a state supreme court to admit to membership of its bar an otherwise qualified person on the sole ground that he had conscientious scruples against war and would not use force to prevent wrong under any circumstances. Since he could not, so the justices of the state court found, swear in good faith to uphold the state constitution, which requires service in the militia in time of war, we held that refusal to permit him to practice law did not violate the First Amendment, as its commands are incorporated in the Due Process Clause of the Fourteenth Amendment. Again, the relation between the obligations of membership in the bar and service required by the state in time of war, the limited effect of the state’s holding upon speech and assembly, and the strong interest which every state court has in the persons who become officers of the court were thought sufficient to justify the state action. See also Hamilton v. Regents, supra.
[406]*406It is contended that the principle that statutes touching First Amendment freedoms must be narrowly drawn dictates that a statute aimed at political strikes should make the calling of such strikes unlawful but should not attempt to bring about the removal of union officers, with its attendant effect upon First Amendment rights. We think, however, that the legislative judgment that interstate commerce must be protected from a continuing threat of such strikes is a permissible one in this case. The fact that the injury to interstate commerce would be an accomplished fact before any sanctions could be applied, the possibility that a large number of such strikes might be called at a time of external or internal crisis, and the practical difficulties which would be encountered in detecting illegal activities of this kind are factors which are persuasive that Congress should not be powerless to remove the threat, not limited to punishing the act. We recently said that “nothing in the Constitution prevents Congress from acting in time to prevent potential injury to the national economy from becoming a reality.” North American Co. v. Securities & Exchange Commission, 327 U. S. 686, 711 (1946). While this statement may be subject to some qualification, it indicates the wide scope of congressional power to keep from the channels of commerce that which would hinder and obstruct such commerce.
VI.
Previous discussion has considered the constitutional questions raised by § 9 (h) as they apply alike to members of the Communist Party and affiliated organizations and to persons who believe in overthrow of the Government by force. The breadth of the provision concerning belief in overthrow of the Government by force would raise additional questions, however, if it were read [407]*407very literally to include all persons who might, under any conceivable circumstances, subscribe to that belief.
But we see no reason to construe the statute so broadly. It is within the power and is the duty of this Court to construe a statute so as to avoid the danger of unconstitutionality if it may be done in consonance with the legislative purpose. United States v. Congress of Industrial Organizations, 335 U. S. 106, 120-121 (1948); United States v. Delaware & Hudson Co., 213 U. S. 366, 407-408 (1909). In enacting § 9 (h), Congress had as its objective the protection of interstate commerce from direct interference, not any intent to disturb or proscribe beliefs as such. Its manifest purpose was to bring within the terms of the statute only those persons whose beliefs strongly indicate a will to engage in political strikes and other forms of direct action when, as officers, they direct union activities. The congressional purpose is therefore served if we construe the clause, “that he does not believe in, and is not a member of or supports any organization that believes in or teaches, the overthrow of the United States Government by force or by any illegal or unconstitutional methods,” to apply to persons and organizations who believe in violent overthrow of the Government as it presently exists under the Constitution as an objective, not merely a prophecy. Congress might well find that such persons — those who believe that the present form of the Government of the United States should be changed by force or other illegal methods — would carry that objective into their conduct of union affairs by calling political strikes designed to weaken and divide the American people, whether they consider actual overthrow of the Government to be near or distant. It is to those persons that § 9 (h) is intended to apply, and only to them. We hold, therefore, that the belief identified in § 9 (h) is a belief in the objective of overthrow by force or by any illegal or unconstitutional [408]*408methods of the Government of the United States as it now exists under the Constitution and laws thereof.
As thus construed, we think that the “belief” provision of the oath presents no different problem from that present in that part of the section having to do with membership in the Communist Party. Of course we agree that one may not be imprisoned or executed because he holds particular beliefs. But to attack the straw man of “thought control” is to ignore the fact that the sole effect of the statute upon one who believes in overthrow of the Government by force and violence — and does not deny his belief— is that he may be forced to relinquish his position as a union leader. That fact was crucial in our discussion of the statute as it relates to membership in the Communist Party. To quote, with pertinent substitutions, an apt statement of that principle, post, p. 434: “The Act does not suppress or outlaw the [belief in overthrow of the Government], nor prohibit it or [those who hold that belief] from engaging in any aboveboard activity .... No individual is forbidden to be or to become a philosophical [believer in overthrow of Government] or a full-fledged member of [a group which holds that belief]. No one is penalized for writing or speaking in favor of [such a belief] or its philosophy. Also, the Act does not require or forbid anything whatever to any person merely because he is [a believer in overthrow of the Government by force]. It applies only to one who becomes an officer of a labor union.”
If the principle that one may under no circumstances be required to state his beliefs on any subject nor suffer the loss of any right or privilege because of his beliefs be a valid one, its application in other possible situations becomes relevant. Suppose, for example, that a federal statute provides that no person may become a member of the Secret Service force assigned to protect the President unless he swears that he does not believe in assassination [409]*409of the President. Is this beyond the power of Congress, whatever the need revealed by its investigations? An affirmative answer hardly commends itself to reason unless, indeed, the Bill of Rights has been converted into a “suicide pact.” Terminiello v. Chicago, 337 U. S. 1, 37 (1949) (dissenting opinion). Yet the example chosen is far-fetched only because of the manifest absurdity of reliance upon an oath in such a situation. One can have no doubt that the screening process in the selection of persons to occupy such positions probes far deeper than mere oath-taking can possibly do.
To hold that such an oath is permissible, on the other hand, is to admit that the circumstances under which one is asked to state his belief and the consequences which flow from his refusal to do so or his disclosure of a particular belief make a difference. The reason for the difference has been pointed out at some length above. First, the loss of a particular position is not the loss of life or liberty. We have noted that the distinction is one of degree, and it is for this reason that the effect of the statute in proscribing beliefs — like its effect in restraining speech or freedom of association — must be carefully weighed by the courts in determining whether the balance struck by Congress comports with the dictates of the Constitution. But it is inaccurate to speak of § 9 (h) as “punishing” or “forbidding” the holding of beliefs, any more than it punishes or forbids membership in the Communist Party.
Second, the public interest at stake in ascertaining one’s beliefs cannot automatically be assigned at zero without consideration of the circumstances of the inquiry. If it is admitted that beliefs are springs to action, it becomes highly relevant whether the person who is asked whether he believes in overthrow of the Government by force is a general with five hundred thousand men at his command or a village constable. To argue that because the latter [410]*410may not be asked his beliefs the former must necessarily be exempt is to make a fetish of beliefs. The answer to the implication that if this statute is upheld “then the power of government over beliefs is as unlimited as its power over conduct and the way is open to force disclosure of attitudes on all manner of social, economic, moral and political issues,” post, p. 438, is that that result does not follow “while this Court sits.” 20 The circumstances giving rise to the inquiry, then, are likewise factors to be weighed by the courts, giving due weight, of course, to the congressional judgment concerning the need. In short, the problem of balancing the conflicting individual and national interests involved is no different from the problem presented by proscriptions based upon political affiliations.
Insofar as a distinction between beliefs and political affiliations is based upon absence of any “overt act” in the former case, it is relevant, if at all, in connection with problems of proof. In proving that one swore falsely [411]*411that he is not a Communist, the act of joining the Party is crucial. Proof that one lied in swearing that he does not believe in overthrow of the Government by force, on the other hand, must consist in proof of his mental state. To that extent they differ.
To state the difference, however, is but to recognize that while objective facts may be proved directly, the state of a man’s mind must be inferred from the things he says or does. Of course we agree that the courts cannot “ascertain the thought that has had no outward manifestation.” But courts and juries every day pass upon knowledge, belief and intent — the state of men’s minds — having before them no more than evidence of their words and conduct, from which, in ordinary human experience, mental condition may be inferred. See 2 Wigmore, Evidence (3d ed.) §§ 244, 256 et seq. False swearing in signing the affidavit must, as in other cases where mental state is in issue, be proved by the outward manifestations of state of mind. In the absence of such manifestations, which are as much “overt acts” as the act of joining the Communist Party, there can be no successful prosecution for false swearing.21
Considering the circumstances surrounding the problem — the deference due the congressional judgment concerning the need for regulation of conduct affecting interstate commerce and the effect of the statute upon rights of speech, assembly and belief — we conclude that § 9 (h) [412]*412of the National Labor Relations Act, as amended by the Labor Management Relations Act, 1947, does not unduly infringe freedoms protected by the First Amendment. Those who, so Congress has found, would subvert the public interest cannot escape all regulation because, at the same time, they carry on legitimate political activities. Cf. Valentine v. Chrestensen, 316 U. S. 52 (1942). To encourage unions to displace them from positions of great power over the national economy, while at the same time leaving free the outlets by which they may pursue legitimate political activities of persuasion and advocacy, does not seem to us to contravene the purposes of the First Amendment. That Amendment requires that one be permitted to believe what he will. It requires that one be permitted to advocate what he will unless there is a clear and present danger that a substantial public evil will result therefrom. It does not require that he be permitted to be the keeper of the arsenal.
VII.
There remain two contentions which merit discussion. One is that § 9 (h) is unconstitutionally vague. The other is that it violates the mandate of Art. I, § 9 of the Constitution that “No Bill of Attainder or ex post facto Law shall be passed.”
The argument as to vagueness stresses the breadth of such terms as “affiliated,” “supports” and “illegal or unconstitutional methods.” There is little doubt that imagination can conjure up hypothetical cases in which the meaning of these terms will be in nice question. The applicable standard, however, is not one of wholly consistent academic definition of abstract terms. It is, rather, the practical criterion of fair notice to those to whom the statute is directed. The particular context is all important.
The only criminal punishment specified is the application of § 35 (A) of the Criminal Code, 18 U. S. C. § 1001, which covers only those false statements made [413]*413“knowingly and willfully.” The question in any criminal prosecution involving a non-Communist affidavit must therefore be whether the affiant acted in good faith or knowingly lied concerning his affiliations, beliefs, support of organizations, etc. And since the constitutional vice in a vague or indefinite statute is the injustice to the accused in placing him on trial for an offense, the nature of which he is given no fair warning, the fact that punishment is restricted to acts done with knowledge that they contravene the statute makes this objection untenable. As this Court pointed out in United States v. Hagen, 314 U. S. 513, 524 (1942), “A mind intent upon willful evasion is inconsistent with surprised innocence.” Cf. Omaechevarria v. Idaho, 246 U. S. 343 (1918); Hygrade Provision Co. v. Sherman, 266 U. S. 497 (1925); Screws v. United States, 325 U. S. 91 (1945). Without considering, therefore, whether in other circumstances the words used in § 9 (h) would render a statute unconstitutionally vague and indefinite, we think that the fact that under § 35 (A) of the Criminal Code no honest, untainted interpretation of those words is punishable removes the possibility of constitutional infirmity.
The unions’ argument as to bill of attainder cites the familiar cases, United States v. Lovett, 328 U. S. 303 (1946); Ex parte Garland, 4 Wall. 333 (1867); Cummings v. Missouri, 4 Wall. 277 (1867). Those cases and this also, according to the argument, involve the proscription of certain occupations to a group classified according to belief and loyalty. But there is a decisive distinction: in the previous decisions the individuals involved were in fact being punished for past actions; whereas in this case they are subject to possible loss of position only because there is substantial ground for the congressional judgment that their beliefs and loyalties will be transformed into future conduct. Of course, the history of the past conduct is the foundation for the judgment as to what [414]*414the future conduct is likely to be; but that does not alter the conclusion that § 9 (h) is intended to prevent future action rather than to punish past action.
This distinction is emphasized by the fact that members of those groups identified in § 9 (h) are free to serve as union officers if at any time they renounce the allegiances which constituted a bar to signing the affidavit in the past. Past conduct, actual or threatened by their previous adherence to affiliations and beliefs mentioned in § 9 (h), is not a bar to resumption of the position. In the cases relied upon by the unions on the other hand, this Court has emphasized that, since the basis of disqualification was past action or loyalty, nothing that those persons proscribed by its terms could ever do would change the result. See United States v. Lovett, supra, at p. 314; Cummings v. Missouri, supra, at p. 327. Here the intention is to forestall future dangerous acts; there is no one who may not, by a voluntary alteration of the loyalties which impel him to action, become eligible to sign the affidavit. We cannot conclude that this section is a bill of attainder.
In their argument on this point, the unions seek some advantage from references to English history pertinent to a religious test oath. That experience is written into our Constitution in the following provision of Article VI: “The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.” It is obvious that not all oaths were abolished; the mere fact that § 9 (h) is in oath form hardly rises to the stature of a constitutional objection. All that was forbidden was a “religious Test.” We do not think that the oath [415]*415here involved can rightly be taken as falling within that category.
Clearly the Constitution permits the requirement of oaths by officeholders to uphold the Constitution itself. The obvious implication is that those unwilling to take such an oath are to be barred from public office. For the President, a specific oath was set forth in the Constitution itself. Art. II, § 1. And Congress has detailed an oath for other federal officers.22 Obviously, the Framers of the Constitution thought that the exaction of an affirmation of minimal loyalty to the Government was worth the price of whatever deprivation of individual freedom of conscience was involved. All that we need hold here is that the casting of § 9 (h) into the mold of an oath does not invalidate it, if it is otherwise constitutional.
We conclude that § 9 (h) of the National Labor Relations Act, as amended by the Labor Management Relations Act, 1947, as herein construed, is compatible with the Federal Constitution and may stand. The judgments of the courts below are therefore
Affirmed.
Mb. Justice Douglas, Mr. Justice Clark and Mr. Justice Minton took no part in the consideration or decision of these cases.