In the Timaeus, Plato makes a distinction between reason and necessity. This distinction is often accounted for as a distinction between two types of causation: purpose oriented causation and mechanistic causation. While reason is associated with the soul and taken to bring about its effects with the good and the beautiful as the end, necessity is understood in terms of a set of natural laws pertaining to material things. In this paper I shall suggest that there are reasons to reconsider the latter part of this account and argue for a non-mechanistic understanding of necessity. I will first outline how the notion of necessity is introduced in the dialogue. Next I will show how a mechanistic account of necessity fails to capture Plato’s purpose of treating it as a causal factor; and, finally, I will argue that this purpose is better understood as an attempt, on Plato’s part, to account for the causal origin of disorder and irrationality, an origin articulated in terms of a pre-cosmic situation and the notoriously difficult notion of the third kind.