How can ethical truth be verified? How can we be sure that ethical ideals are more than fine wishes, expressing subjective aspiration, but having no counterpart in the ultimate constitution of things? This is the dark doubt that haunts the minds of ethical writers, as well as of the average man. We ask to have the things we believe in, the objects of our supreme aspiration, verified. How can they be verified? I think that we shall see light in this matter once we have grasped the thought that verification, both in science and in ethics, is nothing more than exemplification. In the case of causality, in science, verification does not consist in mere recurrence. For if we find, even by a single carefully guarded experiment, that a given phenomenon A is the true antecedent of B, then we take leave to predict that B will always follow A, without regard to the repetition of the sequence in our experience.[39] Indeed, no amount of repetition would justify prediction. The problem in the case of causality is to determine the true antecedent and the true consequent. For at any moment there are innumerable phenomena that might possibly be antecedents of B. How obtain certainty that A is the causal antecedent? By the synthetic process. We assume a unity, say energy. We assume that there are differentiæ, say a certain mathematically determined quantum of mechanical energy in A, and a determined quantum of thermal energy in B. No sooner have these differentiæ been mathematically determined, than in virtue of the assumed unity of energy underlying the differences, we pronounce the nexus to be necessary. We predict that B will always follow A. Causality, therefore, is an example of a synthesis which over-arches sequences. The fact that the phenomena are sequent does not affect the principle involved. Whenever we contemplate an example of synthesis, that is, defined differentiæ of some sort, and a defined underlying unity of some sort, the mind affirms that reality exists. There are degrees of reality. The degree of completeness with which the synthetic function is carried out in any instance determines the degree.