Any facts may be a subject of science, if they follow one another according to constant laws; and this, whether, although the ultimate laws are known, yet, of the derivative laws on which a phenomenon directly depends, either _none_, as in Meteorology, or, as in Tidology, _only_ the laws of the greater causes on which the chief part of a phenomenon directly depends, have been ascertained, and not those of all the minor modifying causes; or, as in Astronomy (which is therefore called an _exact_ science), both the ultimate laws are known, and also the derivative laws as well of the greater as of all the minor causes. The science of Human Nature cannot be exact, the causes of human conduct being only approximately known. Hence it is impossible to predict _with scientific accuracy_ any one man's acts, resulting as they do partly from his circumstances, which, in the future, cannot be precisely foreseen, and, partly, from his character, which can never be exactly calculated, because the causes which have determined it are sure, in the aggregate, not to be entirely like those which have determined any other man's. But approximate generalisations, though only probably true as to the acts and characters of individuals, will be certainly true as to those of masses, whose conduct is determined by general causes chiefly; and they are therefore sufficient for political and social science. They must, however, be connected deductively with the universal laws of human nature on which they rest, or they will be only low empirical laws. This is the text of the next two chapters. CHAPTER IV. THE LAWS OF MIND.