Great article explaining some profound details about ownership:

http://www.coindesk.com/bitcoin-ownership-impact-fungibility/

When you aquire a stolen article from a thief, you might be responsible, and have to give the article back to the rightful owner.

Not so with money, when you sell a can of beer to a known thief, nobody can claim the money from you. The reasoning behind it is to secure the fungibility of the money. When you receive money in a transaction, you should not worry about the source of that money. Exactly what we need for bitcoin, for the same reason.

Come to think of it, this is not so with the banks, they worry a lot where the money comes from, and I  have to declare by which acceptable means I have come to own them. Are deposit money not fungible? Strange world, indeed.