On the Principles of Nature 3

Since some things can be, although they are not, and some things now are; those which can be and are not are said to be in potency, but those which are said to be in act. But existence is twofold: one is essential existence or the substantial existence of a thing, for example man exists, and this is existence simpliciter.†1 The other is accidental existence, for example man is white, and this is existence secundum quid

Nevertheless, one is sometimes used for the other: Every cause can be called a principle and every principle a cause.

23. However, cause seems to add something to principle as commonly used, because that which is a principle,†1 whether the existence of a posterior follows from it or not, can be a principle; for example the manufacturer is called the principle of the knife because the existence of the knife comes from his operation. But, when something is moved from whiteness to blackness, whiteness is said to be the principle †2 of that motion; and universally, everything from which motion begins is called a principle.†3 However, whiteness is not that from whose existence the existence of a posterior, in this case blackness, follows. Hence we say that a cause is that from whose existence another follows. Therefore that principle from which motion begins cannot really be called a cause, even though it may be called a principle. Because of this, privation is placed among the principles and not among the causes, because privation is that from which generation begins. But it can also be called a per accidens cause insofar as it is coincident with matter, as was said above.... Thus it is plain from what was said that principle, in some way, applies to more than does cause, and cause to more than does element. This is what the Commentator says in the fifth book of the Metaphysics.