Chapter I.
Schopenhauer points out that Plato and Kant agree in recommending, as the method of all knowledge, obedience to two laws:—that of Homogeneity, and that of Specification. The former bids us, by attention to the points of resemblance and agreement in things, get at their kinds, and combine them into species, and these species again into genera, until we have arrived at the highest concept of all, that which embraces everything. This law being transcendental, or an essential in our faculty of reason, assumes that nature is in [pg 478] harmony with it, an assumption which is expressed in the old rule: Entia præter necessitatem non esse multiplicanda. The law of Specification, on the other hand, is stated by Kant in these words: Entium varietates non temere esse minuendas. That is to say, we must carefully distinguish the species which are united under a genus, and the lower kinds which in their turn are united under these species; taking care not to make a leap, and subsume the lower kinds and individuals under the concept of the genus, since this is always capable of division, but never descends to the object of pure perception. Plato and Kant agree that these laws are transcendental, and that they presuppose that things are in harmony with them.
The previous treatment of the principle of sufficient reason, even by Kant, has been a failure, owing to the neglect of the second of these laws. It may well be that we shall find that this principle is the common expression of more than one fundamental principle of knowledge, and that the necessity, to which it refers, is therefore of different kinds. It may be stated in these words: Nihil est sine ratione cur potius sit, quam non sit. This is the general expression for the different forms of the assumption which everywhere justifies that question “Why?” which is the mother of all science.