Secondly, is the principle of form self-explanatory? Here, again, we must answer negatively. Most of what was said of Plato under this head applies equally to Aristotle. Plato asserted that the Absolute is reason, and it was therefore incumbent on him to show that his account of reason was truly rational. He failed to do so. Aristotle asserts the same thing, for form is only {336} another word for reason. Hence he must show us that this form is a rational principle, and this means that he must show us that it is necessary. But he fails to do so. How is form a necessary and self-determining principle? Why should there be such a principle as form? We cannot see any necessity. It is a mere fact. It is nothing but an ultimate mystery. It is so, and that is an end of it. But why it should be so, we cannot see. Nor can we see why there should be any of the particular kinds of form that there are. To explain this, Aristotle ought to have shown that the forms constitute a systematic unity, that they can be deduced one from another, just as we saw that Plato ought to have deduced all the Ideas from one another. Thus Aristotle asserts that the form of plants is nutrition, of animals sensation, and that the one passes into the other. But even if this assertion be true, it is a mere fact. He ought not merely to have asserted this, but to have deduced sensation from nutrition. Instead of being content to allege that, as a fact, nutrition passes into sensation, he ought to have shown that it must pass into sensation, that the passage from one to the other is a logical necessity. Otherwise, we cannot see the reason why this change occurs. That is to say, the change is not _explained_.