In Chapter 5 of his 2nd Treatise of Government, John Locke makes an extremely important argument concerning the origin of property rights. His argument has 4 main premises. First, Locke begins by assuming that the Earth is originally held in common by all men. Second, Locke says that, though he has no property in a pre-human all-natural world, man does have sole ownership of property in his own person to which no one else has any claim. Further, the activity of labor falls into this category of property in man's person. Third, Locke believes that mixing one's personal property with un-owned or natural substances improves, or at least alters, the substance and "annexes" an owned piece of the laborer to that substance. It is also an implicit premise, from the conclusion which Locke draws, that he believes that the act of mixing personal property with natural substances removes the raw substance from its natural state, and, thus, from the field of potential objects which other men can use or appropriate or own.
From all of this Locke deduces that labor leads to the creation of property. Labor upon raw substances improves their use-value, or, at the very least, takes them out of the state in which nature originally provides them, and, because labor is something which the laborer owns, and because that labor is mixed with the raw substance, the laborer comes to own the fruits of his labor. Furthermore, due to Locke's first premise, the laborer must not take however much he wants or can, but only enough so that there is "enough and as good" left for everyone else. This is because the Earth is held in common by all men, so all deserve an original fair share or, at least, fair chance at a share, of it.
Locke's argument is a deductive argument. He provides several factual (at least to him) premises and from them, logically deduces his conclusion. One might argue that premise three is, in fact, an incorrect induction of a universal principle from several observations. If Locke were to have only said that labor improves raw materials, this would be true. However, in stating that labor is not solely improvement of raw materials, but can be a simple removal of them from their natural state, Locke in fact merely states a new factual premise.
Locke's argument is perfectly logically valid. The difficulty with the argument comes not with the logical validity of his deduction, but with the factual validity of the premises upon which Locke bases his deduction. Marx's thought, in particular, seemed to take issue with the idea that a man is the sole owner of his labor, because labor and production are social phenomena. Further, the idea of "mixture of labor" is, as many critics have pointed out, a rather vague idea, and yet, it is the premise upon which his whole argument hinges. If Locke's argument is going to be disproven, it must be challenged on the ground of his premises, and, primarily, on the grounds of the idea that a man has sole ownership over his labor, and the idea that labor, as personal property, mixes with raw materials in some way.