This task involves a number of important circumstances. First of all must be considered the manner in which the man on the street conceives the causal relation between different objects. The notion of causality, as Schwarz[132] shows, is essentially foreign to the man on the street. He is led mainly by the analogy of natural causality with that of human activity and passivity, e.g., the fire is active with regard to water, which simply must sizzle passively. This observation is indubitably correct and significant, but I think Schwarz wrong to have limited his description to ordinary people; it is true also of very complex natures. It is conceivable that external phenomena shall be judged in analogy with the self, and inasmuch as the latter often appears to be purely active, it is also supposed that those natural phenomena which appear to be especially active are really so. In addition, many objects in the external world with which we have a good deal to do, and are hence important, do as a matter of fact really appear to be active--the sun, light, warmth, cold, the weather, etc., so that we assign activity and passivity only according to the values the objects have for us. The ensuing mistake is the fact that we overlook the alternations between activity and passivity, or simply do not make the study such alternations require; yet the correct apportionment of action and reaction is, for us, of greatest importance. In this regard, moreover, there is always the empty problem as to whether two things may stand in causal relation,--empty, because the answer is always yes. The scientific and practical problem is as to whether there exists an actual causal nexus. The same relation occurs in the problem of reciprocal influences. No one will say, for example, that any event exercises a reciprocal influence on the sun, but apart from such relatively few cases it would not only be supposed that A is the cause of the effect B, but also that B might have reciprocally influenced A. Regard for this possibility may save one from many mistakes.