A _categorical_ proposition, and an _hypothetical_ answering it, do both signify the same, if the propositions be necessary; but not if they be contingent. For example, if this, _every man is a living-creature_, be true, this also will be true, _if any thing be a man, the same is also a living-creature_; but in contingent propositions, though this be true, _every crow is black_, yet this, _if any thing be a crow, the same is black_, is false. But an _hypothetical_ proposition is then rightly said to be true, when the consequence is true, as _every man is a living-creature_, is rightly said to be a true proposition, because of whatsoever it is truly said _that is a man_, it cannot but be truly said also, _the same is a living creature_. And therefore whensoever an _hypothetical_ proposition is true, the _categorical_ answering it, is not only true, but also necessary; which I thought worth the noting, as an argument, that philosophers may in most things reason more solidly by _hypothetical_ than _categorical_ propositions. [Sidenote: The same proposition diversely pronounced.] 12. But seeing every proposition may be, and uses to be, pronounced and written in many forms, and we are obliged to speak in the same manner as most men speak, yet they that learn philosophy from masters, had need to take heed they be not deceived by the variety of expressions. And therefore, whensoever they meet with any obscure proposition, they ought to reduce it to its most simple and categorical form; in which the copulative word _is_ must be expressed by itself, and not mingled in any manner either with the subject or predicate, both which must be separated and clearly distinguished one from another. For example, if this proposition, _man can not sin_, be compared with this, _man cannot sin_, their difference will easily appear if they be reduced to these, _man is able not to sin_, and, _man is not able to sin_, where the predicates are manifestly different. But they ought to do this silently by themselves, or betwixt them and their masters only; for it will be thought both ridiculous and absurd, for a man to use such language publicly. Being therefore to speak of _equipollent_ propositions, I put in the first place all those for _equipollent_, that may be reduced purely to one and the same categorical proposition. [Sidenote: Propositions that may be reduced to the same categorical proposition, are equipollent.] 13. Secondly, that which is categorical and necessary, is _equipollent_ to its hypothetical proposition; as this categorical, _a right-lined triangle has its three angles equal to two right angles_, to this hypothetical, _if any figure be a right-lined triangle, the three angles of it are equal to two right angles_. [Sidenote: Universal propositions converted by contradictory names, are equipollent.]