Full Idea
We understand something simpliciter when we think we know of the explanation because of which the object holds that it is its explanation, and also that it is not possible for it to be otherwise.
Gist of Idea
We understand a thing when we know its explanation and its necessity
Source
Aristotle (Posterior Analytics [c.327 BCE], 71b10)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.2
A Reaction
The second half sounds odd, since we ought to understand that something could have been otherwise, and knowing whether or not it could have been otherwise is part of the understanding. It sounds like Spinozan determinism.