Chapter 5 examines the development of the logical basis required for natural philosophy in Plato. In particular it shows how Plato in the Sophist develops further understanding not only of negation and the connection operator, but also, in connection with this, the principle of non-contradiction. These developments allow for connecting Being and non-Being, which is necessary for making sense of motion without falling into inconsistencies. The chapter then examines Plato’s employment of the principle of sufficient reason and the criterion of rational admissibility in the Timaeus. He develops the principle of sufficient reason further by distinguishing for the first time between necessary and rational reasons. And rational admissibility is taken up by Plato in the way used by the atomists: that is, the basic ontological constituents not only have to be testable by our own reason, but they also have to explain the phenomena. These requirements together emerge as Plato’s standards for natural philosophy and cosmology by being the positive criteria an eikôs mythos has to fulfil.