Locke's approach to property rights was an early attempt at a definition, demonstrating that we can own a house, property, some land that we worked, but we can't own other people.

There are naturally flaws in his definitions of property, yet he was one of the first philosphers to clarify a consistent position against human slavery. Socrates dared at most to suggest that Greeks should not enslave fellow Greeks, and that was perhaps bordering a controversial stance at his time.