It follows from this twofold analysis that the principle of causality involves two contradictory conceptions of duration, two mutually exclusive ways of prefiguring the future in the present. Sometimes all phenomena, physical or psychical, are pictured as _enduring_ in the same way, and therefore in the way that _we_ do: in this case the future will exist in the present only as an idea, and the passing from the present to the future will take the form of an effort which does not always lead to the realization of the idea conceived. Sometimes, on the other hand, duration is regarded as the characteristic form of conscious states; in this case, things are no longer supposed to _endure_ as we do, and a mathematical pre-existence of their future in their present is admitted. Now, each of these two hypotheses, when taken by itself, safeguards human freedom; for the first would lead to the result that even the phenomena of nature were contingent, and the second, by attributing the necessary determination of physical phenomena to the fact that things do not _endure_ as we do, invites us to regard the self which is subject to duration as a free force. Therefore, every clear conception of causality, where we know our own meaning, leads to the idea of human freedom as a natural consequence. Unfortunately, the habit has grown up of taking the principle of causality in both senses at the same time, because the one is more flattering to our imagination and the other is more favourable to mathematical reasoning. Sometimes we think particularly of the regular _succession_ of physical phenomena and of the kind of inner effort by which one _becomes_ another; sometimes we fix our mind on the absolute _regularity_ of these phenomena, and from the idea of regularity we pass by imperceptible steps to that of mathematical necessity, which excludes duration understood in the first way. And we do not see any harm in letting these two conceptions blend into one another, and in assigning greater importance to the one or the other according as we are more or less concerned with the interests of science. But to apply the principle of causality, in this ambiguous form, to the succession of conscious states, is uselessly and wantonly to run into inextricable difficulties. The idea of force, which really excludes that of necessary determination, has got into the habit, so to speak, of amalgamating with that of necessity, in consequence of the very use which we make of the principle of causality in nature. On the one hand, we know force only through the witness of consciousness, and consciousness does not assert, does not even understand, the absolute determination, now, of actions that are still to come: that is all that experience teaches us, and if we hold by experience we should say that we feel ourselves free, that we perceive force, rightly or wrongly, as a free spontaneity. But, on the other hand, this idea of force,