Full Idea
The cosmos is visible, tangible and corporeal, and therefore perceptible by the senses; therefore it is an object of opinion and sensation, and therefore change and coming into being.
Gist of Idea
If the cosmos is an object of perception then it must be continually changing
Source
Plato (Timaeus [c.362 BCE], 28d)
Book Reference
Plato: 'Timaeus and Critias', ed/tr. Lee,Desmond [Penguin 1971], p.41