Full Idea
The other way of enquiry, that IT IS NOT, and IT is bound NOT TO BE, cannot be explored, for you could neither recognise nor express that which IS NOT.
Gist of Idea
The realm of necessary non-existence cannot be explored, because it is unknowable
Source
Parmenides (fragments/reports [c.474 BCE], B02), quoted by Simplicius - On Aristotle's 'Physics' 9.116.28-
Book Reference
'Ancilla to the Pre-Socratic Philosophers', ed/tr. Freeman,Kathleen [Harvard 1957], p.42