Ok, lets make this a little easier for you. You seem to have a hard time with knife and probable cause. Lets assume the knife was indeed legal for Freddie the drug dealer to carry.

Mosby’s asserts that the police failed to establish probable cause for Gray’s arrest on the basis that she later determined Gray’s knife (the basis for the arrest) to have been legal. As reported by the New York Times, Mosby claims that the arresting officers:

"failed to establish probable cause for Mr. Gray’s arrest, as no crime had been committed."

This is simply not how probable cause works in the context of an arrest .

Here’s what probable cause does not mean: That the officer making an arrest has determined that the facts on which the arrest is founded are true to an absolute certainty, and that the arrest becomes illegal should these reasonably believed “facts” later turn out to be untrue or mistaken.

This should be self-evident by the inclusion of the word “probable” in the phrase “probable cause.”

Arrests often go uncharged, and charges are often dismissed, and defendants are even acquitted at trial, often based upon a later conclusion that the underlying facts which suggested a crime are untrue or mistaken. None of these outcomes makes the initial arrest illegal.

When an officer makes an arrest based on an articulable statement of probable cause, that arrest becomes illegal only if the officer knew or reasonably should have known that no crime had, in fact, been committed.

In short, police are entitled to make reasonable mistakes, and such a reasonable mistake does not make the arrest illegal.

Such a mistake may, of course, make further prosecution of the offense impractical or outright unjust. But that’s a completely different matter than whether an arrest was unlawful.

In the Gray instance, the arresting officer may in fact have been mistaken about whether a spring-assisted knife falls within the statutory prohibition on possession of a switchblade, or on whether Gray’s possession of the knife while (presumably) on probation was an offense subject to arrest.

But unless he actually knew or reasonably should have known either of those possibilities to be the case, legally-speaking probable cause for the arrest existed, and the arrest itself was not a crime.

Again, if the officer were mistaken the arrest may be defective for purposes of further prosecution. But this does not mean that an officer is limited to making an arrest only in circumstances where criminal conduct is a legal and moral certainty.

A society in which this were required, or permitted would be, I expect, a society that most Americans would find an unpleasant place to live.

Thus, even if it turns out that the knife was legal and that Freddie Gray were legally permitted to possess the knife under the circumstances, if the police officer reasonably believed that either of these were offenses subject to arrest, and neither knew or should have known that this belief was incorrect at the time of the arrest, then there existed probable cause for the arrest, and the arrest itself is entirely lawful from the context of that officer’s conduct.