There are two conditions to consider. One is the strict difference between what is causally related and its accidental concomitants,--a difference with regard to which experience is so often misleading, for two phenomena may occur together at the same time without being causally connected. When a man is ninety years old and has observed, every week in his life, that in his part of the country there is invariably a rainfall every Tuesday, this observation is richly and often tested, yet nobody will get the notion of causally connecting Tuesday and rain--but only because such connection would be regarded as generally foolish. If the thing, however, may be attributed to coincidence with a little more difficulty, then it becomes easier to suppose a causal connection; e.g., as when it rains on All-souls day, or at the new moon. If the accidental nature of the connection is still less obvious, the observation becomes a much-trusted and energetically defended meteorological law. This happens in all possible fields, and not only our witnesses but we ourselves often find it very difficult to distinguish between causation and accident. The only useful rule to follow is to presuppose accident wherever it is not indubitably and from the first excluded, and carefully to examine the problem for whatever causal connection it may possibly reveal. “Whatever is united in any perception must be united according to a general rule, but a great deal more may be present without having any causal relation.” The second important condition was mentioned by Schopenhauer:[134] “As soon as we have assigned causal force to any great influence and thereby recognized that it is efficacious, then its intensification in the face of any resistance according to the intensity of the resistance will produce finally the appropriate effect. Whoever cannot be bribed by ten dollars, but vacillates, will be bribed by twenty-five or fifty.”