But there will be another occasion in our lectures to deal more fully with the logic of experiment and with the differences of descriptive knowledge and real rational science. THE THREE DIFFERENT TYPES OF KNOWLEDGE ABOUT NATURE Natural sciences cannot originate before the given phenomena of nature have been investigated in at least a superficial and provisional manner, by and for the practical needs of man. But as soon as true science begins in any limited field, dealing, let us say, with animals or with minerals, or with the properties of bodies, it at once finds itself confronted by two very different kinds of problems, both of them--like all “problems”--created in the last resort by the logical organisation of the human mind, or, to speak still more correctly, of the Ego. In any branch of knowledge which practical necessities have separated from others, and which science now tries to study methodically, there occur general sequences in phenomena, general orders of events. This uniformity is revealed only gradually, but as soon as it has shown itself, even in the least degree, the investigator seizes upon it. He now devotes himself chiefly, or even exclusively, to the generalities in the sequences of all changes. He is convinced that there must be a sort of most general and at the same time of most universal connection about all occurrences. This most universal connection has to be found out; at least it will be the ideal that always will accompany the inquiring mind during its researches. The “law of nature” is the ideal I am speaking about, an ideal which is nothing less than one of the postulates of the possibility of science at all. Using for our purposes a word which has been already introduced into terminology by the philosopher Windelband, though in a somewhat different sense, we shall call that part of every branch of natural sciences which regards the establishment of a law of nature as its ideal, “nomothetic,” *i.e.* “law-giving.” But while every natural science has its nomothetic side, it also has another half of a very different kind. This second half of every natural science does not care for the same general, the same universal, which is shown to us in every event in a different and specified kind: it is diversity, it is specification, that constitutes the subject of its interest. Its aim is to find a sufficient reason for the types of diversities, for the types of specifications. So in chemistry there has been found a systematic order in the long series of the compounds and of the elements; crystallography also has its different systems of crystals, and so on. We have already employed the word by which we shall designate this second half of every natural science: it is the “systematic” side of science.