We now come to the definitions by content. They have been numerous, but we shall only quote a few. The most usual consists in saying, that _Psychology studies the facts of consciousness_. This formula passes, in general, as satisfactory. The little objection raised against it is, that it excludes the unconscious facts which play so important a part in explaining the totality of mental life; but it only requires some usual phrase to repair this omission. One might add, for instance, to the above formula: conscious facts and those which, while unconscious under certain conditions, are yet conscious in others.
This is not, however, the main difficulty, which is far more serious. On close examination, it is seen that the term, _fact of consciousness_, is very elastic, and that for a reason easy to state. This is, that all facts which exist and are revealed to us reach us by the testimony of the consciousness, and are, consequently, facts of consciousness. If I look at a locomotive, and analyse its machinery, I act like a mechanic; if I study under the microscope the structure of infusoria, I practise biology; and yet the sight of the locomotive, the perception of the infusoria, are just facts of consciousness, and should belong to psychology, if one takes literally the above definition, which is so absolute that it absorbs the entire world into the science of the mind. It might, indeed, be remarked that certain phenomena would remain strictly psychological, such as, for instance, the emotions, the study of which would not be disputed by any physical science; for the world of nature offers us nothing comparable to an emotion or an effort of will, while, on the other hand, everything which is the object of physical science--that is, everything which can be perceived by our external senses--may be claimed by psychology. Therefore, it is very evident the above definition is much too wide, and does not agree with _solo definito_. It does not succeed in disengaging the essential characteristic of physics. This characteristic indeed exists, and we foresee it, but we do not formulate it.
Another definition by content has not been much more happy. To separate the material from the moral, the conception of Descartes was remembered, and we were told that: "Psychology is the science of what exists only in time, while physics is the science of what exists at once in time and in space."