In Johnson v. United States, 333 U.S. 10, 92 L. Ed. 436, 68 S. Ct. 367 (1948), a confidential informant informed the police that opium was being used by "unknown persons" in the Europe Hotel. When they arrived at the hotel to further investigate, the police smelled the "distinctive and unmistakable" odor of burning opium coming from a certain room. They did not know the occupants of the room, so they knocked and announced their presence. The defendant then opened the door, and the police asked for admittance. A search of the room revealed opium and a smoking apparatus. Mr. Justice Jackson wrote for the Court: At the time entry was demanded the officers were possessed of evidence which a magistrate might have found to be probable cause for issuing a search warrant . . . . If the presence of odors is testified to before a magistrate and he finds the affiant qualified to know the odor, and it is one sufficiently distinctive to identify a forbidden substance, this Court has never held such a basis insufficient to justify issuance of a search warrant . . . . The point of the Fourth Amendment, which often is not grasped by zealous officers, is not that it denies law enforcement the support of the usual inferences which reasonable men draw from evidence. Its protection consists in requiring that those inferences be drawn by a neutral and detached magistrate instead of being judged by the officer engaged in the often competitive enterprise of ferreting out crime . . . . Crime, even in the privacy of one's own quarters, is, of course, of grave concern to society, and the law allows such crime to be reached on proper showing. The right of officers to thrust themselves into a home is also a grave concern, not only to the individual but to a society which chooses to dwell in reasonable security and freedom from surveillance. When the right of privacy must reasonably yield to the right of search is, as a rule, to be decided by a judicial officer, not by a policeman or Government enforcement agent.Johnson v. United States, 333 U.S. at 13-14.