But as for those common received opinions, _Nature abhors vacuity_, _Nature doth nothing in vain_, and the like, which are neither evident in themselves, nor at all to be demonstrated, and which are oftener false than true, they are much less to be acknowledged for principles. To return, therefore, to definitions; the reason why I say that the cause and generation of such things, as have any cause or generation, ought to enter into their definitions, is this. The end of science is the demonstration of the causes and generations of things; which if they be not in the definitions, they cannot be found in the conclusion of the first syllogism, that is made from those definitions; and if they be not in the first conclusion, they will not be found in any further conclusion deduced from that; and, therefore, by proceeding in this manner, we shall never come to science; which is against the scope and intention of demonstration. [Sidenote: The nature & definition of a definition.] 14. Now, seeing definitions (as I have said) are principles, or primary propositions, they are therefore speeches; and seeing they are used for the raising of an _idea_ of some thing in the mind of the learner, whensoever that thing has a name, the definition of it can be nothing but the explication of that name by speech; and if that name be given it for some compounded conception, the definition is nothing but a resolution of that name into its most universal parts. As when we define man, saying _man is a body animated, sentient, rational_, those names, _body animated_, _&c._ are parts of that whole name _man_; so that definitions of this kind always consist of _genus_ and _difference_; the former names being all, till the last, _general_; and the last of all, _difference_. But if any name be the most universal in its kind, then the definition of it cannot consist of _genus_ and _difference_, but is to be made by such circumlocution, as best explicateth the force of that name. Again, it is possible, and happens often, that the _genus_ and _difference_ are put together, and yet make no definition; as these words, _a straight line_, contain both the _genus_ and _difference_; but are not a definition, unless we should think a straight line may be thus defined, _a straight line is a straight line_: and yet if there were added another name, consisting of different words, but signifying the same thing which these signify, then these might be the definition of that name. From what has been said, it may be understood how a definition ought to be defined, namely, _that it is a proposition, whose predicate resolves the subject, when it may; and when it may not, it exemplifies the same_. [Sidenote: Properties of a definition.] 15. The properties of a definition are: