Well, we suppose that we _know_ things (in the strict and proper sense of the word) when we suppose ourselves to know the cause by reason of which the thing is to be the cause of it; and that this cannot be otherwise. It is plain that the idea intended to be conveyed by the term _knowing_ is something of this kind; because they who do not really know suppose themselves thus related to the matter in hand and they who do know really are so that of whatsoever there is properly speaking Knowledge this cannot be otherwise than it is Whether or no there is another way of knowing we will say afterwards, but we do say that we know through demonstration, by which I mean a syllogism apt to produce Knowledge, i.e. in right of which through having it, we know. If Knowledge then is such as we have described it, the Knowledge produced by demonstrative reasoning must be drawn from premisses _true_ and _first_, and _incapable of syllogistic proof_, and _better known_, and _prior in order of time_, and _causes of the conclusion_, for so the principles will be akin to the conclusion demonstrated. (Syllogism, of course there may be without such premisses, but it will not be demonstration because it will not produce knowledge). _True_, they must be, because it is impossible to know that which is not. _First_, that is indemonstrable, because, if demonstrable, he cannot be said to _know_ them who has no demonstration of them for knowing such things as are demonstrable is the same as having demonstration of them. _Causes_ they must be, and _better known_, and _prior_ in time, _causes_, because we then know when we are acquainted with the cause, and _prior_, if causes, and _known beforehand_, not merely comprehended in idea but known to exist (The terms prior, and better known, bear two senses for _prior by nature_ and _prior relatively to ourselves_ are not the same, nor _better known by nature_, and _better known to us_ I mean, by _prior_ and _better known relatively to ourselves_, such things as are nearer to sensation, but abstractedly so such as are further Those are furthest which are most universal those nearest which are particulars, and these are mutually opposed.) And by _first_, I mean _principles akin to the conclusion_, for principle means the same as first And the principle or first step in demonstration is a proposition incapable of syllogistic proof, _i.e._ one to which there is none prior. Now of such syllogistic principles I call that a θέσις which you cannot demonstrate, and which is unnecessary with a view to learning something else. That which is necessary in order to learn something else is an Axiom.