[nybble41]

> The other guy can just 'take' your stamps. As they don't belong to you.

He could, but he won't—not unless he is so much stronger than you that he has no need to worry about reciprocation—because there are things that he thinks of as "his" and doesn't want you to take. It's easier to just tacitly agree, between the two of you, that some things are yours and some things are his, and any changes in that status ought to be worked out on mutually acceptable terms.

This is why private property is one aspect of natural law. The concept arises naturally wherever there exist scarce resources to be allocated for individual use. Societies which recognize this trend and align themselves with it consistently tend to prosper, while others which either ignore it or actively reject the concept of private property struggle to rise above mere subsistence.