Let us realize, for instance, the process which takes place when reason understands something it did not know before. Think of some peculiar, unexpected and unknown chemical transformation which takes place suddenly and without apparent cause in some mixture. Assume furthermore that the same reaction takes place more frequently after that, until experience demonstrates that this inexplicable change occurs whenever sunlight touches the mixture. This already constitutes a certain understanding of the process. Assume furthermore that subsequent experience teaches us that several other substances have the faculty of producing the same reaction in connection with sunlight. We have then arranged the new reaction in line with a number of phenomena of the same class, that is to say we have enlarged, deepened, completed our understanding of it still more. And if we finally discover that a special part of the sunlight unites with a special element of the mixture and thereby produces this new reaction, we have generalized this experience, or experienced this generalization, in a "pure" state, in other words, the theory of this reaction is complete, reason has solved its problem, and yet it has done nothing more than it did when it classified the animal and vegetable kingdoms in families, genera, species, etc. To find the species, the genus, the sex, etc., of anything means to understand it. Reason proceeds in the same way when it investigates the causes of certain transformations. Causes are, in the last instance, not noticed and furnished by means of sight, hearing, feeling, not by means of the sense perceptions. They are rather supplied by the faculty of thought. It is true, causes are not the "pure" products of the faculty of thought, but are produced by it in connection with sense perceptions and their material objects. This raw material gives the objective existence to the causes produced by the mind. Just as we demand that a truth should be the truth about some objective phenomenon, so we also demand that a cause should be real, that it should be the cause of some objective effect. The understanding of any concrete cause is conditioned on the empirical study of its material, while the understanding of any general cause is based on the study of the faculty of reason. In the understanding of concrete causes, the material of study varies, but reason maintains a constant or general attitude. The cause, as a general cause, is a pure conception, and it is based on the study of the multiformity of concrete understandings of causes, or on the multiplied study of concrete causes. Hence we are compelled to return to the concrete material of the general concept, to the understanding of concrete causes, if we wish to analyze the concept of a general cause.