156.] But this unity of the double cause is also (β) actual. All this alternation is properly the cause in act of constituting itself and in such constitution lies its being. The nullity of the distinctions is not only potential, or a reflection of ours (§ 155). Reciprocal action just means that each characteristic we impose is also to be suspended and inverted into its opposite, and that in this way the essential nullity of the 'moments' is explicitly stated. An effect is introduced into the primariness; in other words, the primariness is abolished: the action of a cause becomes reaction, and so on. Reciprocal action realises the causal relation in its complete development. It is this relation, therefore, in which reflection usually takes shelter when the conviction grows that things can no longer be studied satisfactorily from a causal point of view, on account of the infinite progress already spoken of. Thus in historical research the question may be raised in a first form, whether the character and manners of a nation are the cause of its constitution and its laws, or if they are not rather the effect. Then, as the second step, the character and manners on one side and the constitution and laws on the other are conceived on the principle of reciprocity: and in that case the cause in the same connexion as it is a cause will at the same time be an effect, and _vice versâ._ The same thing is done in the study of Nature, and especially of living organisms. There the several organs and functions are similarly seen to stand to each other in the relation of reciprocity. Reciprocity is undoubtedly the proximate truth of the relation of cause and effect, and stands, so to say, on the threshold of the notion; but on that very ground, supposing that our aim is a thoroughly comprehensive idea, we should not rest content with applying this relation. If we get no further than studying a given content under the point of view of reciprocity, we are taking up an attitude which leaves matters utterly incomprehensible. We are left with a mere dry fact; and the call for mediation, which is the chief motive in applying the relation of causality, is still unanswered. And it we look more narrowly into the dissatisfaction felt in applying the relation of reciprocity, we shall see that it consists in the circumstance, that this relation, instead of being treated as an equivalent for the notion, ought, first of all, to be known and understood in its own nature. And to understand the relation of action and reaction we must not let the two sides rest in their state of mere given facts, but recognise them, as has been shown in the two paragraphs preceding, for factors of a third and higher, which is the notion and nothing else. To make, for example, the manners of the Spartans the cause of their constitution and their constitution conversely the cause of their manners, may no doubt be in a way correct. But, as we have comprehended neither the manners nor the