Full Idea
Anaxagoras says that intellect (which is a cause of change) is not affected by or mixed in with anything else; for this is the only way in which it can cause change, while being itself changeless, and control things without mixing with them.
Gist of Idea
Anaxagoras says mind remains pure, and so is not affected by what it changes
Source
report of Anaxagoras (fragments/reports [c.460 BCE]) by Aristotle - Physics 256b24
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.202
A Reaction
I suggest that this is the germ of the original concept of freewill - of the mind as somehow outside the causal processes of the world, so that it can initiate change without itself being affected by other causes. Aristotle says he's right; I disagree.