Now the Greek word θεωρεῖν means originally to behold, to be a spectator of. But in psychological parlance, it was used to designate the contemplation of things in their relations, the grasp of them in their order, the knowledge of their laws. A theoretical knowledge of anything is properly, therefore, a systematic knowledge, a knowledge of them as systematically interrelated, a knowledge of their principles of connection. But this is exactly what every true science is. Hence every science is, as science, theoretical. To call a science theoretical is to commit a tautology, but, as with many tautologies, a useful end is gained. The attention is riveted by the tautological adjective on an essential characteristic, which may easily be, and in this case has been, ignored. All science, to repeat, is theoretical; it is a matter of seeing, not of doing, and no new mark is added to the concept of science by calling it theoretical, but the danger is averted of supposing that a science may not be theoretical.
The fact, however, that all science is theoretical, does not preclude some sciences from being practical, provided that by practical is meant what it often means, namely, facilitating practice, advantageous in practice, having a direct bearing upon practice. Thus a practical suggestion is one that helps to the doing of something desired; a practical idea is one that aids in the accomplishment of some purpose; a practical rule is one the observance of which is of value in the performance of some work. Now it requires but little reflection to observe that very much of our knowledge is directly practical in this sense. It is not itself a doing of some work, unless we call the activity of knowing itself a work, but it is indispensable for the performance of the complicated work which men have to do. This is so obviously true that it is now often maintained that knowledge is to be accounted for as a result of natural selection. This means that knowledge as a psychic phenomenon is so useful to the knower that by its help he wins in the struggle for existence, and is thereby enabled to propagate his kind, transmitting to his descendants his useful power of knowing. Thus we have what has been called the doctrine of the bionomic value of knowledge, and the advocates of this doctrine include within it also the asser-
