Miscellanies 8.3.7

But if the former, then by asking for a demonstration of each demonstration we shall go on to infinity, and so demonstration will be overturned; but if the latter, these very things that are credible of themselves will become the starting points of demonstrations. Indeed, the philosophers agree that the first principles of all things are undemonstrable. So if there is demonstration at all, there must of all necessity first be something credible of itself, which is called first and undemonstrable. All demonstration, then, is traced back to undemonstrated faith. There may also be other starting points of demonstrations after the source that springs from faith: the things plainly evident to sense and to thought. For the things that present themselves to sense are simple and not further analyzable; those that present themselves to thought are simple, rational, and first; and the compound things generated from them are no less evident and credible, and more rational than the first. Of what follows and what conflicts, then, … is the power proper to reason, which we all have by nature. If, then, an account is found such that it can supply conviction to what is not yet credible from what is already credible, we shall say that this very thing is the essence of demonstration. It has been said that the kind that belongs to conviction and the kind that belongs to demonstration are both twofold: the one produces only persuasion in the souls of the hearers, the other produces scientific knowledge. If, then, one begins from what is evident to sense and thought, and then draws the proper conclusion, one truly demonstrates; but if one begins only from reputable opinions, and not from first things, that is, from things evident neither to sense nor to thought, then if one draws the proper conclusion one will syllogize, but will not make the demonstration scientific; and if not the proper conclusion, one will not even syllogize at all.