Some of the best modern scholastics account for the difference between cause and principle in the following manner: A principle, they say, is conceived to differ from a cause in two things: first, in this, that a cause always precedes its effect by priority of nature,[144] whereas a principle does not require such a priority; secondly, in this, that the cause does not communicate its own identical nature to its effect, whereas the principle can communicate its own identical nature to that which it principiates.[145] From these two differences a third one might be gathered, viz., that the effect has always a real dependence from[146] its cause, whilst the thing principiated does not always really depend from its principles.[147] These grounds of distinction between principles and causes have been thought of, with the avowed object of paving the way to explain how the Eternal Father can be the principle, without being the cause, of his Eternal Son, and how the Father and the Son can be the principle, without being the cause, of the Holy Ghost.
But we must observe that there are four genera of causes and of principles: the efficient, the material, the formal, and the final; and that the two differences alleged by these writers between principle and cause do not apply to principles and causes of the same genus, but are applicable only when some principle belonging to one genus is wrongly compared with some cause pertaining to another genus.
That there are four genera of causes we will take for granted, as it is the universal doctrine of philosophers. That there are also four genera of principles corresponding to the four genera of causes is evident; for every cause must contain within itself the principle of its causality; and, in fact, Aristotle himself clearly affirms that there are as many causes as principles, and that all causes are also principles,[148] in the sense which we have already explained. Lastly, that the two aforesaid differences between principle and cause do not apply to principles and causes of the same genus can be easily verified by a glance at each genus. Let the reader take notice of the following statements, and then judge for himself.
The efficient cause (the agent) and the efficient principle (its active power) are both, by priority of nature, prior to the thing produced or principiated, and both have a nature numerically distinct from that of the thing produced or principiated.
In the same manner the final cause (the object willed) and the finalizing principle (the known goodness and desirability of the object) both are, by priority of nature, prior to the act caused or principiated, and both have a nature numerically distinct from that of the act caused or principiated.
Thus, also, the material cause (actual matter) and the material principle (the passiveness of matter) are both, by priority of nature, prior to the thing effected or principiated, and both identify themselves with the thing effected or principiated.
Accordingly, with regard to these three kinds of causation and principiation, it is quite impossible to admit that the difference between a cause and a principle is to be accounted for by a recourse to the two aforementioned grounds of distinction, so long as the causes and principles, which are compared, belong to one and the same genus.
As to the formal cause and the formal principle, we shall presently see that they are not distinct things; but, even if we were disposed to consider them as distinct, such a distinction could not possibly rest on the two grounds of which we have been speaking; for the formal cause and the formal principle have no priority of nature[149] with respect to the thing caused or principiated, and both identify themselves with the same. We are, therefore, satisfied that the opinion which we have criticised has no foundation in truth.
Let us, then, resume our previous explanation, and see how the difficulty above proposed against its completeness can be solved. We have shown that the notion of cause implies the notion of principle, together with that of a subject to which the principle belongs. We must, therefore, admit that a principle differs from a cause of the same genus, as an incomplete or metaphysical entity differs from a complete or physical being; or, in other terms, that a real cause, rigorously speaking, is a complete being, which gives origin to an effect; whilst a real principle, properly speaking, is only that through which the cause gives origin to its effect. The cause is id quod causat;[150] the principle is id quo causa causat.[151]