Thomas Reid uses the term 'moral liberty' to refer to a kind of free will that is agent-causal and incompatible with determinism. I offer and textually support a new interpretation of Reid's third argument for moral liberty, which Reid presents in Section 4.8 of Essays on the Active Powers of Man. Generally regarded as obscure, most commentators either ignore Reid's third argument or lend it cursory attention. In my interpretation, Reid points to the truism that we have reason to think (...) that human persons conceive of long-term plans. Then, Reid argues that determinism implies that God both conceives of and enacts these plans, leaving us without any reason to believe that people even conceive of these plans. Therefore, we should hold onto the truism and reject determinism. On my interpretation, Reid employs the premises of a theistic argument from design as premises of his argument. (shrink)