Full Idea
Since he is forced to be guided by appearances, he assumes that the one exists from the viewpoint of reason, but that a plurality exists from the viewpoint of the sense, and so he posits two principles and causes - hot and cold.
Gist of Idea
Reason sees reality as one, the senses see it as many
Source
comment on Parmenides (fragments/reports [c.474 BCE], A24) by Aristotle - Metaphysics 986b27-
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.62
A Reaction
A profound thought. Empiricists emphasies experience, and end up with fragmented reality. Reason explains experience, and in the process sees the world as unities (like objects), though a single unity is going too far.