Full Idea
Parmenides would not agree with anything unless it seemed necessary, whereas his predecessors used to come up with unsubstantiated assertions.
Gist of Idea
Parmenides was much more cautious about accepting ideas than his predecessors
Source
comment on Parmenides (fragments/reports [c.474 BCE], A28) by Simplicius - On Aristotle's 'Physics' 9.116.2-
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.56
A Reaction
from Eudemus