Book 1
7
And what further is attempted in reasoning? To admit the logical consequence of whatever you have properly granted. Well, and is it enough merely to know this necessity? It is not; but we must learn how it happens that such a thing is the consequence of such another, and when one thing follows from one premise, and when from many premises. Is it not moreover necessary that he who would acquit himself skilfully in reasoning should both himself demonstrate whatever he asserts and be able to comprehend the demonstrations of others, and not be deceived by such as use sophistry as if they were reasoning fairly? Hence arises the use and practice of logical forms; and it appears to be indispensable.
