monstrative science, since it is not possible to demonstrate the conclusion of necessity, because accident may possibly not be present, for I speak of accident of this kind. Still some one may perhaps doubt why we must make such investigations about these things, if it is not necessary that the conclusion should be, for it makes no difference if any one interrogating casual things should afterwards give the conclusion: nevertheless we must interrogate not as if (the conclusion) were necessary on account of things interrogated, but because it is necessary for him who asserts these should assert this, and that he should speak truly if the things are truly inherent.
Since, however, whatever are inherent per se are necessarily inherent in every genus, and so far as each is, it is clear that scientific demonstrations are of things "per se" inherent, and consist of such as these. For accidents are not necessary: wherefore it is not necessary to know the conclusion why it is, nor if it always is, but not "per se," as, for instance, syllogisms formed from signs. For what is "per se" will not be known "per se," nor why it is, and to know why a thing is, is to know through cause, wherefore the middle must "per se" be inherent in the third, and the first in the middle.
Chapter 7
It is not therefore possible to demonstrate passing from one genus to another, as, for instance,
