Full Idea
To think is the same as the thought that IT IS, for you will not find thinking without Being, on which it depends for its expression.
Gist of Idea
Thinking implies existence, because thinking depends on it
Source
Parmenides (fragments/reports [c.474 BCE], B08 ll.?), quoted by Simplicius - On Aristotle's 'Physics' 9.145.1-
Book Reference
'Ancilla to the Pre-Socratic Philosophers', ed/tr. Freeman,Kathleen [Harvard 1957], p.44