I. NOTION OF PRINCIPLE.
By the name of principle philosophers designate that whence anything originally proceeds in any manner whatever: Id, unde aliquid quomodocumque procedit. This definition implies that there are many different manners of proceeding, and consequently many different kinds of principles. And so it is. Aristotle, however, shows that principles of all kinds can be reduced to three classes; that is, to those principles of which a thing consists, those through which or out of which a thing is made, and those by which a thing is known: Primum, unde aliquid est, aut fit, aut cognoscitur.[139]—Arist. Metaph. 5.
The first class comprises the principles through which a thing is, viz., by which the thing is intrinsically constituted. These principles are called constituent or intrinsic principles, and are always present by their own entity in the thing principiated; as the matter in the body, and the soul in the animal.
The second class contains the principles through which a thing is made. These principles serve to account for the origin of the thing, and are called extrinsic principles, because they are not present by their own entity in the thing principiated. Thus, the motive power of the sun is not, by its own entity, in the planets to which it imparts movement, but in the sun only; and the medical art is not in the person who has been cured through it, but in the doctor. There is, however, in the planets something proceeding from the motive power of the sun, and in the person cured something proceeding from the medical art, as every one will acknowledge. Whence it is obvious that the extrinsic principles by their very principiation must leave some mark or vestige of themselves in the thing principiated.
The third class consists of those principles through which any conclusion is made known. These principles are general truths, which are made to serve for the demonstration of some other truth, and are called principles of science.
Among the principles of this third class we do not reckon the principles from which the first apprehension and immediate intuition of things proceeds; to wit, either the power through which the object makes an impression on the cognoscitive faculty, or the faculty itself through which the object is apprehended. Our reason is that these principles, thus considered, do not form a class apart. The power of the object to make its impression on the subject is an extrinsic principle of knowledge, and ranks with the principles of the second class above mentioned; whilst the power of the subject to perceive through the intelligible species is an intrinsic principle of knowledge, as well as the species which it expresses within itself, and, therefore, is to be ranked among the principles of the first class. Accordingly, the third class is exclusively made up of those principles which serve for the scientific demonstration of truth; and this is what Aristotle himself insinuates, at least negatively, as he gives no instance of principles of this third class but the premises by which any conclusion is made known.
Before we advance further, we have to remark that, in metaphysics, the first principles of science are assumed, not as a subject of investigation, but as the fundamental base of scientific demonstration. Thus, the principles, Idem non potest simul esse et non esse,[140] Non datur effectus sine causa,[141] Quæ sunt eadem uni tertio sunt eadem inter se,[142] and such like, though usually styled "metaphysical" principles, are not the subject of metaphysical investigation, but are simply presupposed and admitted on the strength of their immediate and incontrovertible evidence. Such principles are perfectly known before all metaphysical disquisition, and need not be traced to other principles. On the other hand, metaphysics, which is the science of reality, deals only with the principles of real beings; whence it follows that the principles of demonstration, which, like the conclusion deduced therefrom, exist in the intellect alone (and therefore are beings of reason, and principiate nothing but other beings of reason), are not comprised in the object of metaphysical inquiry. Hence, the only principles which metaphysics is bound to investigate are those that belong to the first and second class above mentioned; that is, the intrinsic and the extrinsic principles of things: Primum unde aliquid est, and primum unde aliquid fit.[143]
Principles and causes are often confounded, although it is well known that they are not identical. Hence, our next question is: In what does a cause differ from a principle?
It is commonly admitted that all causes are principles, but not all principles causes; which evidently implies that a cause is something more than a principle. In fact, when we use the word "cause," we wish to designate a being in which we know that there is a principle of causation; whence it is evident that the common notion of cause implies the notion of principle, and something else besides—that is, the notion of a subject to which the principle belongs. Thus, we say that the moon causes the tides by its attractive power; the moon is the cause, and the attractive power is its principle of causation. In like manner, we say that an orator causes great popular emotion by his eloquence; the orator is the cause, and his eloquence is his principle of causation.
From these instances it would be easy to conclude that the difference between a cause and a principle lies in this, that the cause is a complete being, whilst the principle is only an appurtenance of the cause. But as we know from theology that there are principles which cannot be thus related to causes, we cannot consider the above as an adequate and final answer to the question proposed.