(1) The same reasons which lead us to demand a cause A for any event B, and to find that cause in an assemblage of antecedent events, require that A should be similarly determined by another assemblage of antecedent events, and that this cause of A should itself have its own antecedent cause, and so on indefinitely. Thus the causal principle, logically applied, never yields an intelligible explanation of any event. Instead of exhibiting the transition A-B as the logical expression of a coherent principle, it refers us for the explanation of this transition to a previous instance of the same kind of transition, and then to another, and so forth without end. But it is impossible that what is not intelligible in one instance should become intelligible by the mere multiplication of similar unintelligibilities. (2) Similarly if we look within the transition A-B. This transition being continuous must have its intermediate stages. A becomes B because it has already become C, and the transition A-C-B is again “explained” by showing that A became D which became C which became E which became B. And each of the stages A-D, D-C, C-E, E-B can be once more submitted to the same sort of analysis. But in all this interpolation of intermediate stages there is nothing to show the nature of the common principle in virtue of which the stages form a single process. We are, in fact, trying to do what we try to do wherever we establish a relation between terms, to answer a question by repeating it. And we decided at the end of our last chapter that this kind of repetition is never an answer to any question. How entirely it fails to answer the question we ask whenever we look for a cause is obvious. We want to know why B exists, and we are told that B exists because it is determined by the previous existence of A. But why does A exist? Because of the previous existence of C. And so ultimately the existence of everything depends on the existence of something else, and this again on the existence of still something else. If this is so, since nothing can exist until its cause has existed, and the cause again not until its cause has existed, then, as this unending series has no first term, nothing can ever come into existence at all. This inevitable introduction of the indefinite regress whenever we try to think out the causal principle to its logical consequences, has sometimes been treated as proving the inherent defectiveness of the human mind. What it proves rather is that Causality is not a proper formulation of the real principle of the unity of all experience.