Appendix: The principle of sufficient reason In connection with the intelligibility (which is the meaning of the truth) of being many authors discuss a principle not found in so many words in St. Thomas, that of sufficient reason, usually stated as "every being has a sufficient reason." The principle, in its modern development, must be credited to the rationalism of Leibniz, since when it has loomed large in many philosophical minds. And for all that one searches St. Thomas in vain for mention of it, the principle does bear an interpretation (though not the Leibnizian one) that fits it fairly into his thought.
The first approach is by way of another axiom, that "every being is true," which, no one doubts, is authentic St. Thomas. Every being, this means, is by its very nature ordered or proportioned to an intellect; hence "every being is true" has the meaning of "every being is intelligible." Not every being, however, is intelligible through and through, or perfectly. The intelligibility in question is perfect only if the being in question is perfect, or perfectly being; which, of course, is God, perfectly intelligible in himself - not, need we say it, to us. Created beings, compacted as it were of being and nonbeing, are in some measure shrouded from the intellect; which is to say, to such beings there necessarily clings a greater or lesser degree of opacity, of unintelligibility. Our principle, then, does not mean that from the mere notion of being can be deduced the whole content of reality; nor, more to the point, that all reality yields itself perfectly to every speculative intellect. Such optimism, which characterizes certain forms of philosophical rationalism, is not justified by the facts. Extreme interpretation of this kind will be precluded if we give the principle the fuller formulation that "every being is intelligible so far as it is being."
If being is intelligible, there must be a ground for its intelligibility. This ground is precisely its "sufficient reason," that which both determines being to be and renders it intelligible. Every being, accordingly, is intelligible because for every being there is a sufficient reason, something that adequately accounts for what it is or has. The sufficient reason, to go a step further, may be within a being, flowing as it were from its very nature or essence. The square, for example, or the color red are what they are because of their essence, which constitutes them what they are. This is plain in the case of the square; you cannot define it (which is to give its essence) without mentioning, among other things, its sides. But the sufficient reason of being is not always in a being itself or in its essence. If a man is white, this does not result from his essence, or all men would be white. In such cases, then, the being in question must have its sufficient reason in another, which will be its cause. As St. Thomas puts it, "Whatever belongs to a thing, but not as following from the thing itself, belongs to it through some cause, as white to a man." Which is the sense of,
Omne quod alicui convenit non secundum quod ipsum est, per aliquam causam convenit ei, sicut album homini.And why so? Because, continues St. Thomas, "that which has no cause is something first and immediate; hence it is necessary that it be by reason of itself and in consequence of what it is," - the rendering of,
Quod causam non habet, primum et immediatum est; unde necesse est ut sit per se et secundum quod ipsum.25Being, accordingly, is what it is either by self and by essence, or by another. Whence we conclude the principle as follows:
"Every being, as being, has its sufficient reason in itself or in another."
This formulation, it is worth noting, covers both orders of being; both essence and existence; however, we have already as much as said that the sufficient reason in one order is not the same as in the other. So, in the order of essence the principle says (in effect) that properties have their sufficient reason in the essence of the subject to which they pertain - for example, that the interior angles of a triangle equal two right angles stems from the very nature of this figure, and the fact of man's aptitude for instruction is a consequence of his rational nature. In the order of existence (or concrete being) the sufficient reason is a cause in the more proper sense of the word. Any being which does not exist of itself (hence a contingent being: this stone, this tree, etc.) has its sufficient reason in another as in its cause; and since causality is not all one kind, the sufficient reason will vary according to the line of causality. What we are saying, then, is that the principle of sufficient reason is an analogical principle and that its application must in consequence be analogical, varying with the orders of being and the types of causality. To forget this is to invite the extremest apriorism, a rationalism untempered by realism.26