are seven colors or sixty-four elements. Enumeration is evidently possible in psychology. Thus the assumption of seven colors is mental, not physical. We are said to have five senses, because we are supposed to have five sorts of sensations, not because there are five sorts of sense-organs. The number of harmonious intervals in music is a true psychological constant. The number of equally noticeable differences between the minimum and maximum sensation may turn out to be an important constant. Attention has recently been called to moral statistics, and these are psychological measurements of a sort. Thus, it is something to know that, out of a million mothers, so many are infanticides. The lack of great scientific value consists in the fact that we do not measure or even define the conditions under which this percentage obtains. But if we find the percentage to be so much greater in winter or when the offspring are illegitimate, we are making some progress. There seems to be scientific value in the collection of statistics concerning the inheritance of mental traits, and in other directions. If we say that, when one idea suggests another, there are a certain number of classes among which all cases may be distributed, the distinctions are logical and the number of classes is not of much scientific importance. If, however, after the classes have been defined, we are able to determine the actual proportion of associations falling into each, we are making determinations of some value. If, further, the percentages are found to vary under fixed conditions, according to the occupation of the individual, for example, then such determinations have scientific interest. A still further advance is made, when we are able to correlate the frequency of an association with the time it takes. Here we have, indeed, the beginnings of a mental mechanics.
9. The possibility of measuring time depends on the flow of individual thought.
The motions of the heavenly bodies happen to be so well adapted for the measurement of time that most men will agree
