"Which latter kind of being, that is, the immutable essence, as a distinct thing from individual sensibles, Aristotle plainly asserts against Heraclitus, and those other flowing philosophers in these words: 'We would have these philosophers to know, that besides sensible things that are always mutable, there is another kind of being or entity of such things as are neither subject to motion, corruption, nor generation.' And elsewhere he tells us, that this immovable essence {255} is the object of theoretical knowledge, of the first philosophy, and of the pure mathematics. "Now these immutable entities are the universal _rationes_, or intelligible natures and essences of all things, which some compare to unities, but Aristotle to numbers; which formally considered, are indivisible: saith he, 'The essences of things are like to numbers;' because if but the least thing be added to any number, or subtracted from it, the number is destroyed. "And these are the objects of all certain knowledge. As for example, the objects of geometry are not any individual material triangles, squares, circles, pyramids, cubes, spheres, and the like; which because they are always mutable, nothing can be immutably affirmed of them; but they are those indivisible and unchangeable _rationes_ of a triangle, square, circle; which are ever the same to all geometricians, in all ages and places, of which such immutable theorems as these are demonstrated, as that a triangle has necessarily three angles equal to two right angles.