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The legal concept of possession, as you might know, can be very brief; like opening a bag or turning over a clump of something. "Possession" is just the intent to briefly touch or control a thing -- like intending to touch or turn over a rock. It doesn't mean you discover it first, take time to analyze what it is, think about it, weigh the moral or legal consequences and after a while decide not to possess it. It is just the briefest intent to exercise some minor amount of control over the thing; the person doesn't even have to form the knowledge of specifically what it is.
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