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.
Chapter II.
Schopenhauer in this chapter traces historically the forms in which the principle had been stated by his predecessors, and their influence. He points out that in Greek philosophy it appeared in two aspects—that of the necessity of a ground for a logical judgment, and that of a cause for every physical change—and that these two aspects were systematically confounded. The Aristotelian division, not of the forms of the principle itself, but of one of its aspects, the causal, exemplified a confusion which continued throughout the Scholastic period. Descartes succeeds no better. His proof of the existence of God that the immensity of His nature is a cause or reason beyond which no cause is needed for His existence, simply illustrates the gross confusion between cause [pg 479] and ground of knowledge which underlies every form of this ontological proof. “That a miserable fellow like Hegel, whose entire philosophy is nothing but a monstrous amplification of the ontological proof, should dare to defend this proof against Kant's criticism of it is an alliance of which the ontological proof itself, little as it knows of shame, might well feel ashamed. It is not to be expected I should speak respectfully of people who have brought philosophy into disrespect.” Spinoza made the same confusion when he laid it down that the cause of existence was either contained in the nature and definition of the thing as it existed, or was to be found outside that thing. It was through this confusion of the ground of knowledge with the efficient cause that he succeeded in identifying God with the world. The true picture of Spinoza's “Causa sui” is Baron Munchhausen encircling his horse with his legs, and raising himself and the horse upwards by means of his pigtail, with the inscription “Causa sui” written below. Leibnitz was the first to place the principle of sufficient reason in the position of a first principle, and to indicate the difference between its two meanings. But it was Wolff who first completely distinguished them, and divided the doctrine into three kinds: principium fiendi (cause), principium essendi (possibility), and principium cognoscendi. Baumgarten, Reimarus, Lambert, and Platner added nothing to the work of Wolff, and the next great step was Hume's question as to the validity of the principle. Kant's distinction of the logical or formal principle of knowledge—Every proposition must have its ground; from the transcendental or material principle, Every thing must have its ground—was followed out by his immediate successors. But when we come to Schelling we find the proposition that gravitation is the reason and light the cause of things, a proposition which is quoted simply as a curiosity, for such a piece of nonsense deserves no place among the opinions of earnest and honest inquirers. The chapter concludes by pointing out the futility of the attempts to prove the principle. Every proof is the exhibition of the ground of a judgment which has been expressed, and of which, just because that ground is exhibited, we predicate truth. The principle of [pg 480] sufficient reason is just this expression of the demand for such a ground, and he who seeks a proof, i.e., the exhibition of a ground for this principle itself, presupposes it as true, and so falls into the circle of seeking a proof of the justification of the demand for proof.
Chapter III.
In the third chapter Schopenhauer points out that the two applications of the principle of sufficient reason distinguished by his predecessors, to judgments, which must have a ground, and to the changes of real objects, which must have a cause, are not exhaustive. The reason why the three sides of a certain triangle are equal is that the angles are equal, and this is neither a logical deduction nor a case of causation. With a view to stating exhaustively the various kinds into which the application of the principle falls it is necessary to determine the nature of the principle itself. All our ideas are objects of the subject, and all objects of the subject are our ideas. But our ideas stand to one another as a matter of fact in an orderly connection, which is always determinable a priori in point of form, and on account of which nothing that is in itself separate and wholly independent of other things can be the object of our consciousness. It is this connection which the principle of sufficient reason in its generality expresses. The relations which constitute it are what Schopenhauer calls its root, and they fall into four classes, which are discussed in the four following chapters.