We have yet to ask what is meant by “rule” and what its relation is to probability. Scientifically “rule” means law subjectively taken and is of equal significance with the guiding line for one’s own conduct, whence it follows that there are only rules of art and morality, but no rules of nature. Usage does not imply this interpretation. We say that as a rule it hails only in the daytime; by way of exception, in the night also; the rule for the appearance of whales indicates that they live in the Arctic Ocean; a general rule indicates that bodies that are especially soluble in water should dissolve more easily in warm than in cold water, but salt dissolves equally well in both. Again we say: As a rule the murderer is an unpunished criminal; it is a rule that the brawler is no thief and vice versa; the gambler is as a rule a man of parts, etc. We may say therefore, that regularity is equivalent to customary recurrence and that whatever serves as rule may be expected as probable. If, i.e., it be said, that this or that happens as a rule, we may suppose that it will repeat itself this time. It is not permissible to expect more, but it frequently happens that we mistake rules permitting exceptions for natural laws permitting none. This occurs frequently when we have lost ourselves in the regular occurrences for which we are ourselves responsible and suppose that because things have been seen a dozen times they must always appear in the same way. It happens especially often when we have heard some phenomenon described in other sciences as frequent and regular and then consider it to be a law of nature. In the latter case we have probably not heard the whole story, nor heard general validity assigned to it. Or again, the whole matter has long since altered. Lotze wrote almost half a century ago, that he had some time before made the statistical observation that the great positive discoveries of exact physiology have an average life of about four years. This noteworthy statement indicates that great positive discoveries are set up as natural laws only to show themselves as at most regular phenomena which have no right to general validity. And what is true of physiology is true of many other sciences, even of the great discoveries of medicine, even legal medicine. This, therefore, should warn against too much confidence in things that are called “rules.” False usage and comfortable dependence upon a rule have very frequently led us too far. Its unreliability is shown by such maxims as “Three misses make a rule” or “Many stupidities taken together give a golden rule of life,” or “To-day’s exception is to-morrow’s rule,” or the classical perversion: “The rule that there are no rules without exception is a rule without exception, hence, there is one rule without exception.”