Plato became more conservative in his outlook and more attentive to existing social values and traditions as he aged. The city of the Republic would have required a revolution to set up. In the later Laws, Plato becomes less revolutionary and describes a “second-best” city in which there are traditional families and rulers are elected, rather than specially bred.

In the Parmenides Plato offers a series of criticisms to his earlier theory of forms, which he is apparently unable to answer and which are later taken up by Aristotle. The most famous of these is the “third man argument.” Suppose we discover a form that accounts for what makes similar things similar. For example, every cat is different, but all cats share the same catness because they participate in the cat form. Now, if we compare this form with any one thing that participates in it—in this case, compare your cat with the cat form—the form and the participating thing will have similarities that make it necessary to posit a second form. If we then make comparisons of the cat to the second form, a third form will need to be posited, and on and on and on to an infinite regress. That is, Plato was aware of the theoretical problems with his theory of forms.