The _Causa cognoscendi_, or logical _reason_ for the affirmation of a truth, as distinguished from the psychological factors which lead a particular individual to affirm it, is clearly identical with what modern logicians call the _Ground_. A given proposition must logically be affirmed as true in the last resort, because it fills a place in a wider system of truths which no other proposition would fill. Thus, _e.g._, a special proposition about the relation between the sides and angles of a triangle is logically necessitated, because it is an integral element in the development of a system of geometrical ideas which repose as a whole upon certain fundamental assumptions as to the character of spatial order. The original presuppositions cannot be worked out to their logical consequence in a body of internally coherent geometrical notions unless the proposition in question is included in that body. And reciprocally, the logical justification for regarding these presuppositions rather than any others as sound, lies in the fact that they yield a body of internally consistent consequences. Incidentally, we see by means of this illustration that Ground and Consequence are mutually convertible, which is what we might have inferred from the way in which we defined them as mutually complementary aspects of a single systematic whole. What we are concerned with in the everyday and scientific treatment of Causation, is not this purely logical relation of Ground and Consequence, but something partly identical with it, partly different. The _Causa fiendi_ has no significance except in connection with occurrences or events in time, and may roughly be said to correspond with what Aristotle denotes the “Source of Change”—ἀρχὴ κινήσεως or ὅθεν ἡ κίνησις—and his mediæval followers named the Efficient Cause. Cause, in the popular sense of the word, denotes the attempt to carry out the principle of the interconnection of events in a system along special lines by regarding every event as completely determined by conditions which are themselves previous events. Widely as the popular and the scientific uses of the term “cause” diverge in minor respects, they agree in the essential point. That every event has its cause is understood, both in everyday life and in the sciences which use the concept of causation, to mean that the occurrence and the character of every event in the time-series is completely determined by preceding events. In more technical language, causation for everyday thought and for the sciences means _one-sided dependence_ of the present on the past, and the future on the present.