[128] Sigwart in his _Logik_, Book II. (English translation by Helen Dendy) has made a profound study of the classification of the psychological laws in psychology, and their relative value. He divides them into three categories, according to the nature of the relations which they express: 1. Psychophysical laws which formulate constant relations between states of consciousness and the cerebral states. Ex. the relation between the sensation directly received, and the image that is reproduced in consciousness. 2. Psychological laws properly so-called; these express the internal relations of the states of consciousness. Ex. Law of conservation of impressions, law of association, law of systematisation by volition. 3. Laws expressive of the reciprocal action that human thoughts and volitions exert one upon the other: they presuppose the intervention of social causes, and are to this day vague and ill-determined; hence there are no fixed rules for the government of humanity, or the bringing up of children. [129] “Fundamental laws are, or should be, only the simplest, most abridged, and most economical mode of expressing facts, within the limits of precision possible to our observations and experiences. The laws of nature are simple, essentially because—among all the possible modes of expression—we choose the simplest” (see Mach, _Mechanics_, Chicago, 1893, and _Popular Scientific Lectures_, 23d ed., Chicago, 1898, under the headings “Economy of Thought,” “Law,” etc.). “In formulating a general, simple, precise law, based upon relatively few experiences, which, moreover, present certain divergencies, we only obey a necessity from which the human mind cannot free itself.” (Poincaré.) [130] Since our subject is the tracing out of the _concept_ of law in its different degrees, starting from the generic image, we have no need to study the nature of the laws proper to each science (logic, mathematics, mechanics, physico-chemistry, biology, etc.), nor to discuss their value. For this point, see Boutroux, _L’idée de loi naturelle dans la science et la philosophie contemporaine_. Paris, 1895. [131] Brown, quoted by Quatrefages (_Précurseurs de Darwin_, p. 218), who adds, “If this were the case, one would not find many species denoted by particular names among savages, and our own illiterate population. The general notion of species is on the contrary one of those that are forced upon us, directly we look round. The difficulty is to formulate it clearly, to give it scientific precision, and this is a very real problem.”