Opinion of the Court. 298 U.S. an official body is permitted to take the place of allowable official discretion or to supplant the standing law as a rule of human conduct, the government ceases to be one of laws and becomes an autocracy. Against the threat of such a contingency the courts have always been vigilant, and, if they are to perform their constitutional duties in the future, must never cease to be vigilant, to detect and turn aside the danger at its beginning. The admonition of Mr. Justice Bradley in Boyd v. United States, 116 U. S. 616, 635, should never be forgotten: "It may be that it is the obnoxious thing in its mildest and least repulsive form; but illegitimate and unconstitutional practices get their first footing in that way, namely, by silent approaches and slight deviations from legal modes of procedure. . . . It is the duty of courts to be watchful for the constitutional rights of the citizen, and against any stealthy encroachments thereon. Their motto should be obsta principiis." Arbitrary power and the rule of the Constitution cannot both exist. They are antagonistic and incompatible forces; and one or the other must of necessity perish whenever they are brought into conflict. To borrow the words of Mr. Justice Day-"there is no place in our constitutional system for the exercise of arbitrary power." Garfield v. Goldsby, 211 U. S. 249, 262. To escape assumptions of such power on the part of the three primary departments of the government, is not enough. Our institutions must be kept free from the appropriation of unauthorized power by lesser agencies as well. And if the various administrative bureaus and commissions, necessarily called and being called into existence by the increasing complexities of our modern business and political affairs, are permitted gradually to extend their powers by encroachments-even petty encroachments— upon the fundamental rights, privileges and immunities of the people, we shall in the end, while avoiding the 1 Opinion of the Court. fatal consequences of a supreme autocracy, become submerged by a multitude of minor invasions of personal rights, less destructive but no less violative of constitutional guaranties. Third. The proceeding for a stop order having thus disappeared, manifestly it cannot serve as a basis for the order of the district court compelling petitioner to appear, give testimony, and produce his private books and papers for inspection by the commission. But the commission contends that the order may rest upon the general power to conduct investigations which it says is conferred by 19 (b). The difficulty with that is that the investigation was undertaken for the declared and sole purpose of determining whether a stop order should issue. The first action taken by the commission was on May 20th, four days before the registration was to become effective under the statute. The commission then, after averring that upon reasonable grounds it believed the registration statement was false in material facts, directed that stop-order proceedings be instituted against the statement. It never has averred or directed anything else. This action was followed by a notice containing like recitals of a more detailed character, and calling upon the registrant to appear and show cause why a stop order should not be issued suspending the effectiveness of the statement. It was upon this direction and notice that all subsequent proceedings were had and upon which they must stand or fall. We do not interpret the order of the district court, the substance of which has already been stated, as resting upon a different view. Nothing appears in any of the proceedings taken by the commission to warrant the suggestion that the investigation was undertaken or would be carried on for any other purpose or to any different end than that specifically named. An official inquisition to compel disclosures of fact is not an end, but a means to an end; and it is Opinion of the Court. 298 U.S. an official body is permitted to take the place of allowable official discretion or to supplant the standing law as a rule of human conduct, the government ceases to be one of laws and becomes an autocracy. Against the threat of such a contingency the courts have always been vigilant, and, if they are to perform their constitutional duties in the future, must never cease to be vigilant, to detect and turn aside the danger at its beginning. The admonition of Mr. Justice Bradley in Boyd v. United States, 116 U. S. 616, 635, should never be forgotten: "It may be that it is the obnoxious thing in its mildest and least repulsive form; but illegitimate and unconstitutional practices get their first footing in that way, namely, by silent approaches and slight deviations from legal modes of procedure. . . . It is the duty of courts to be watchful for the constitutional rights of the citizen, and against any stealthy encroachments thereon. Their motto should be obsta principiis." Arbitrary power and the rule of the Constitution cannot both exist. They are antagonistic and incompatible forces; and one or the other must of necessity perish whenever they are brought into conflict. To borrow the words of Mr. Justice Day-"there is no place in our constitutional system for the exercise of arbitrary power." Garfield v. Goldsby, 211 U. S. 249, 262. To escape assumptions of such power on the part of the three primary departments of the government, is not enough. Our institutions must be kept free from the appropriation of unauthorized power by lesser agencies as well. And if the various administrative bureaus and commissions, necessarily called and being called into existence by the increasing complexities of our modern business and political affairs, are permitted gradually to extend their powers by encroachments-even petty encroachmentsupon the fundamental rights, privileges and immunities of the people, we shall in the end, while avoiding the 1 Opinion of the Court. fatal consequences of a supreme autocracy, become submerged by a multitude of minor invasions of personal rights, less destructive but no less violative of constitutional guaranties. Third. The proceeding for a stop order having thus disappeared, manifestly it cannot serve as a basis for the order of the district court compelling petitioner to appear, give testimony, and produce his private books and papers for inspection by the commission. But the commission contends that the order may rest upon the general power to conduct investigations which it says is conferred by 19 (b). The difficulty with that is that the investigation was undertaken for the declared and sole purpose of determining whether a stop order should issue. The first action taken by the commission was on May 20th, four days before the registration was to become effective under the statute. The commission then, after averring that upon reasonable grounds it believed the registration statement was false in material facts, directed that stop-order proceedings be instituted against the statement. It never has averred or directed anything else. This action was followed by a notice containing like recitals of a more detailed character, and calling upon the registrant to appear and show cause why a stop order should not be issued suspending the effectiveness of the statement. It was upon this direction and notice that all subsequent proceedings were had and upon which they must stand or fall. We do not interpret the order of the district court, the substance of which has already been stated, as resting upon a different view. Nothing appears in any of the proceedings taken by the commission to warrant the suggestion that the investigation was undertaken or would be carried on for any other purpose or to any different end than that specifically named. An official inquisition to compel disclosures of fact is not an end, but a means to an end; and it is Opinion of the Court. 298 U.S. a mere truism to say that the end must be a legitimate one to justify the means. The citizen, when interrogated about his private affairs, has a right before answering to know why the inquiry is made; and if the purpose disclosed is not a legitimate one, he may not be compelled to answer. Since here the only disclosed purpose for which the investigation was undertaken had ceased to be legitimate when the registrant rightfully withdrew his statement, the power of the commission to proceed with the inquiry necessarily came to an end. Dissociated from the only ground upon which the inquiry had been based, and no other being specified, further pursuit of the inquiry, obviously, would become what Mr. Justice Holmes characterized as "a fishing expedition . for the chance that something discreditable might turn up" (Ellis v. Interstate Commerce Comm'n, 237 U. S. 434, 445)—an undertaking which uniformly has met with judicial condemnation. In re Pacific Ry. Comm'n, 32 Fed. 241, 250; Kilbourn v. Thompson, 103 U. S. 168, 190, 192, 193, 195, 196; Boyd v. United States, 116 U. S. 616; Harriman v. Interstate Commerce Comm'n, 211 U. S. 407, 419; Federal Trade Comm'n v. American Tobacco Co., 264 U. S. 298, 305–307. In re Pacific Ry. Comm'n involved the power of a Congressional commission to investigate the private affairs, books and papers of officers and employees of certain corporations indebted to the government. That commission called before it the president of one of these corporations, required the production of private books and papers for inspection, and submitted interrogatories which the witness declined to answer. Acting under the statute, the commission sought a peremptory order from the circuit court to compel the witness to answer the interrogatories. The court, consisting of Mr. Justice Field, Circuit Judge Sawyer, and District Judge Sabin, denied the motion of the district attorney for the order |