We do not know exactly what the phrase "probable cause" means, in strict numerical terms. We do, however, know what it does not mean: "probably." That is, probable cause does not--in the context of Fourth Amendment law mean that the police must have evidence sufficient to conclude that a suspect is
probably guilty or that she probably has evidence of a crime hidden inside her home. It is, accordingly, perfectly consistent with the constitutional ban on unreasonable searches and seizures--under existing doctrine--to arrest or
search a person on the basis of suspicion that does not rise to the level of a "preponderance" of the evidence. The arrestee's guilt (or the presence of evidence in a place to be searched) need not be "more likely than not."

The Supreme Court has said that "`[t]he substance of all the definitions' of probable cause `is a reasonable ground for belief of guilt.' And this `means less than evidence which would justify condemnation' or conviction . . . . [I]t has come to mean more than bare suspicion . .. ." The Court has also said, in discussing post-arrest, probable-cause hearings, that "[probable cause] does not require the fine resolution of conflicting evidence that a reasonable-doubt or
even a preponderance standard demands . . . ." Additionally, "only the probability, and not a prima facie showing, of criminal activity is the standard of probable cause . . . ." And finally, the probable-cause standard "does not demand any showing that such a belief be correct or more likely true than false." Though some of the cases are relatively old, the Court has not subsequently retreated from the position--however obliquely stated--that probable cause is something more than bare suspicion but something less than "more probable than not."

Some may quarrel with the legitimacy of this standard and suggest that a preponderance of the evidence would be a more appropriate prerequisite to arrest. Even if one were to adopt a more stringent definition of "probable cause," however, it would nonetheless remain the case that the standard does not rule out--and can be said, in fact, to contemplate--that innocent people
will regularly suffer the indignity and deprivation of an arrest for crimes of which they are completely innocent. This consequence is unavoidable,regardless of the standard, if searches are permissible at all. Indeed, I have elsewhere argued that the Fourth Amendment, in setting out "probable cause" as a limiting principle for searches and seizures, explicitly and inherently balances the privacy and liberty of innocent people against the also-significant
goal of protection of the public from criminal predation. If it were otherwise, it would not be necessary to tolerate error--the law could simply bar all intrusive criminal investigation on the basis of anything less than proof beyond a reasonable doubt (or better, absolute certainty). Such a robust protection for privacy and liberty, however, would come at a great cost--the inability of law enforcement to act to detect and, perhaps even more importantly, prevent criminal behavior about which they do not already have all of the facts.

The Fourth Amendment compromise thus means, necessarily, that police who investigate to the extent that they may, consistent with constitutional limits, will sometimes inflict frightening, humiliating, and unpleasant lawenforcement events--searches and seizures--on innocent people. If we assume that "probable cause" is as close to a preponderance of the evidence as possible, without actually rising to a preponderance, then the doctrinal standard
contemplates that at least fifty percent of those lawfully searched and seized will be innocent and thus undeserving of any intrusion.