This thesis assesses philosophical arguments in favour of patent systems. These come in both consequentialist and deontological forms, the latter of which are the focus of this analysis. One kind of deontological argument is based on the concept of desert. I argue that on any plausible conception of desert, the patent system fails to distribute rewards as well as viable alternative systems could. The other kind of deontological argument claims that inventors are entitled to patent rights over their inventions as an extension of their natural rights, drawing on a Lockean account of the conditions of legitimate appropriation of unowned goods. After a discussion of the metaphysics of invention, and of the nature of the commons, I argue that Locke's conditions are not in fact always trivially satisfied in the case of patents. Furthermore, entitlement-based arguments conclude that because new inventions are unowned, claiming property rights in them involves only the same moral considerations that would apply in the state of nature. I argue that because we are not in the state of nature, pre-existing property rights also need to be taken into account, which conflict with patents. The broad conclusion of this thesis is that none of the plausible deontological arguments for patent systems are sound. The implication is that any justification of must therefore be made in consequentialist terms; this ultimately rests on strong empirical evidence rather than normative arguments alone.