The positive philosophy seeks to put an end to the hoary error that anything more is open to our knowledge than given facts—­phenomena and their relations.  We do not know the essence of phenomena, and just as little their first causes and ultimate ends; we know—­by means of observation, experiment, and comparison—­only the constant relations between phenomena, the relations of succession and of similarity among facts, the uniformities of which we call their laws.  All knowledge is, therefore, relative; there is no absolute knowledge, for the inmost essence of facts, and likewise their origin, the way in which they are produced, is for us impenetrable.  We know only, and this by experience, that the phenomenon A is invariably connected with the phenomenon B, that the second always follows on the first, and call the constant antecedent of a phenomenon its cause.  We know such causes only as are themselves phenomena.  The fact that our knowledge is limited to the succession and coexistence of phenomena is not to be lamented as a defect:  the only knowledge which is attainable by us is at the same time the only useful knowledge, that which lends us practical power over phenomena.  When we inquire into causes we desire to hasten or hinder the effect, or to change it as we wish, or at least to anticipate it in order to make our preparations accordingly.  Such foresight and control of events can be attained only through a knowledge of their laws, their order of succession, their phenomenal causes. Savoir pour prevoir.  But, although the prevision of facts is the only knowledge which we need, men have always sought after another, an “absolute” knowledge, or have even believed that they were in possession of it; the forerunners of the positive philosophy themselves, Bacon and Descartes, have been entangled in this prejudice.  A long intellectual development was required to reach the truth, that our knowledge does not extend beyond the cognition of the succession and coexistence of facts; that the same procedure must be extended to abstract speculation which the common mind itself makes use of in its single actions.  On the other hand, the positive philosophy, notwithstanding its rejection of metaphysics, is far from giving its sanction to empiricism.  Every isolated, empirical observation is useless and uncertain; it obtains value and usefulness only when it is defined and explained by a theory, and combined with other observations into a law—­this makes the difference between the observations of the scholar and the layman.

The positive stage of a science, which begins when we learn to explain phenomena by their laws, is preceded by two others:  a theological stage, which ascribes phenomena to supposed personal powers, and a metaphysical stage, which ascribes them to abstract natural forces.  These three periods denote the childhood, the youth, and the manhood of science.