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In countries with the British legal tradition for criminal law (including the United States), possession is one of the principles of actus reus, the element of a crime that is considered wrong. There is nothing inherently illicit about a sledgehammer, but if it can be demonstrated to have been in the luggage of somebody who had other actus reus to commit a crime (say, had conspired to beat somebody's brains in), then possession could prove guilt or otherwise.

There are degrees of possession. Actual possession is where an item is physically on one's person. Constructive possession is where somebody does not have actual possession, but has knowledge of the presence and nature of an object or substance and has the power and intent to exercise control over the object or substance, either directly or through another person. If the object is in a location that the person has exclusive access to (such as a school locker), then constructive possession can be automatically inferred. If the location is more freely accessible (such as in a public toilet's cistern), then constructive possession could be proven by considering the totality of other circumstances, like other incriminating statements, how visible the location is etc.

Knowing possession means that one is aware that (s)he is in actual or constructive possession of an item, but just to protect people who have drugs planted on them, or little old ladies who board an aircraft forgetting to declare their nail files to security, unwitting possession means one is unaware of being its possessor. Possession can be sole or joint.

Of course, ownership does not imply possession. Neither does a legitimate right to own or use an object imply possession. In the context of criminal law, the owner of a stolen car is not legally responsible for what the thief with actual possession does with it. And being without a driving licence will not excuse the thief.

To prove the principle of possession in a crime, the defendant must be proven to have had actual or constructive possession of an item, and for him/her to be knowledgeable about this fact.