Full Idea
If you understand something demonstratively, it must hold from necessity, so it is plain that your demonstration must proceed through a middle term which is necessary.
Gist of Idea
Demonstrations must be necessary, and that depends on the middle term
Source
Aristotle (Posterior Analytics [c.327 BCE], 75a13)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.11
A Reaction
How can a middle 'term' be necessary, if it is not a proposition? Presumably Socrates is necessarily a man, and men are necessarily mortal, so it is the predication which is necessary.