Full Idea
That which is apprehended by intelligence and reason is always in the same state, but that which is conceived by opinion with the help of sensation and without reason is always in a process of becoming and perishing, and never really is.
Gist of Idea
The apprehensions of reason remain unchanging, but reasonless sensation shows mere becoming
Source
Plato (Timaeus [c.362 BCE], 28a)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.1234
A Reaction
Lots of problems with this, of which I take the main one to be the idea that sensation is 'without reason', as if there were a sharp dichotomy in our ways of evaluating reality. Laws of nature seem to be laws of change, not of stasis.