Necessity, in short, is based upon the principle of logical contradiction; contingency upon that of sufficient reason. Since our actions are in no case necessitated in such a way that their opposite is self-contradictory, or, put positively, since our actions are always determined by the choice of that which seems best, our actions are contingent. Occasionally Leibniz puts the matter in a much simpler way, and one which brings out the essential element more clearly than the foregoing distinction. Some facts are determined by the principle of physical causation; others by that of final causation. Some, in other words, are necessary as the mechanical outcome of their antecedents; others are necessary as involved in the reaching of a given end. It is simply the Aristotelian distinction between efficient and teleological causation. Human action is determined, since it always has a motive or reason; it is contingent, because it springs from this reason and not from its temporal antecedents. It is, in short, determined, but it is also free. It does not require much analysis, however, to see that this distinction, in whatever way it be put, really has no significance, except as it points to the other marks of freedom,--spontaneity and rationality. As we shall see, Leibniz makes and can make no absolute distinction between truths of reason and truths of fact. The contingent and the necessary are one at bottom. To us with our limited intelligence it does indeed often appear as if no contradiction were involved in the former,--as if, for example, a man could turn either to right or left without there being any logical contradiction in either case; but this is because of our defective insight. An intelligence cognizant of the whole matter could see that one action would contradict some truth involved in the constitution of the universe. The source of the contingent and changing is in the necessary and eternal. Thus it is that although Leibniz at one time says that "neither one's self nor any other spirit more enlightened could demonstrate that the opposite of a given action (like going out in preference to staying in) involves contradiction," at another time he says that "a perfect knowledge of all the circumstances, internal and external, would enable any one to foresee" the decision in a given case. If that be so, any other action must be impossible; that is, according to Leibniz's invariable logic, imply contradiction.