The main purpose of this article is to try to understand the connection between the physical universe and the mathematical principles that underlies the cosmological account of the Timaeus. Aristotle's common criticism of Plato's cosmology is that he confuses mathematical and physical constructions. Namely, the Timaeus is the first cosmology founded on mathematical physics. We give a new translation of Timaeus 31b-32b, an important passage to understand the connection between mathematics and physics in Timaeus' cosmological construction. This article is the first of a series about the khôra. We will restrict our focus here to the much-debated question of the primary elements in the khôra, the components of the whole physical world reduced, in an extraordinary elegant construction, to two right triangles.