Plato’s cosmology in the Timaeus can be seen as being framed in biological terms, since it claims the universe as a whole to be a living being, more specifically, a created god. In this chapter, I show that the central assumption that leads Plato to understand the created cosmos as a living being is the idea that the world is as good as possible. In a second step, I want to show that this assumption of the world’s bestness is also responsible for two important twists to the biological framing Plato uses. First, being as good as possible also implies that the world is self-sufficient, which means that many of our common biological notions are of no relevance for an account of the cosmos as a living being. Secondly, I show that while Plato gives an account of all kinds of living beings, his assumption of the bestness of the world leads him to be ultimately interested only in rational living beings. Accordingly, what starts out in biological terms turns into a form of rational psychology and rational theology. This will finally lead to a discussion whether Plato works with a consistent notion of life in the Timaeus.