The purpose of this article is to provide an analysis of the World Soul in Plato's Timaeus. In showing how the description of the World Soul is elaborated in the Timaeus, highlighting the following dimensions, (a) its composition, (b) its mathematical structure, (c) its moving function and (d) its cognitive function, we will establish how this ontological constitution is articulated within the framework of the teleological dimension of the discourse. It will appear that it is the two functions of the World Soul (moving and cognitive) that justify why it possesses the structure of an intermediate mixture constituted from being, sameness and difference. As such, it is the proper cause (aitia), which is exemplified by the Demiurge's teleological deduction in Timaeus' discourse, that determines the auxiliary cause (sunaitia), namely, the structure of the World Soul.