What Knowing is, what Demonstration is, and of what it consists.
We suppose ourselves to know anything absolutely and not accidentally after the manner of the sophists, when we consider ourselves to know that the ground from which the thing arises is the ground of it, and that the fact cannot be otherwise. …
Hence it follows that everything which admits of absolute knowledge is necessary. We will discuss later the question as to whether there is any other manner of knowing a thing, but at any rate we hold that that ‘knowledge comes through demonstration.’ By ‘demonstration’ I mean a scientific syllogism, and by ‘scientific’ a syllogism the mere possession of which makes us know.
If then the definition of knowledge be such as we have stated, the premises of demonstrative knowledge must be true, primary, immediate, better known than, anterior to, and the cause of, the conclusion, for under these conditions the principles will also be appropriate to the conclusion. One may, indeed, have a syllogism without these conditions, but not a demonstration, for it will not produce scientific knowledge. The premises must be true, because it is impossible to know that which is not, e.g., that the diagonal of a square is commensurate with the side. The conclusion must proceed from primary premises that are indemonstrable premises, for one cannot know things of which one can give no demonstration, since to know demonstrable things in any real sense is just to have a demonstration of them. The premises must be Prior, Explanatory, Better known [to us] and Previously cognized; Explanatory, because we only know a thing when we have learned its explanation; Prior, if they are to be explanatory; Previously known not only in our second sense, viz. That their meaning is understood, but that one knows that they are true.