The famous "enthymême" of Descartes, "I think, therefore I am," is a pleonasm. As soon as the human being says to itself "I," the human being affirms its own existence, and distinguishes itself from that external world whence it derives impressions of which it is not the author. {292} In this primary fact are revealed the two primary objects of human knowledge: on the one side the human being himself, the individual person that feels and perceives, that feels himself and perceives himself; on the other side, the external world that is felt and perceived: the subject and the object, (the _moi_ and the _non-moi_.) Such is the twofold field, at the beginning of his intellectual existence, opened to the knowing faculty of man. In each of these fields, whether the human being makes himself or whether he makes the external world the object of his contemplation, he proceeds by the same method; he considers particular facts, classes these under more general facts which serve as their summary, and recognizes laws that govern them, these laws being themselves facts. When this method of observation and of generalization is applied to the outer world, understanding by that world the human body also, it gives birth to the sciences of physics and of physiology. {293} When such method is applied to the human being, regarded as distinct from the body in which he lives and by which he acts, it gives birth to the science of psychology, logic, and morals. It is not here my intention to propose a classification of the sciences, but only to determine the domain of science properly so called--that is to say, the field in which the human mind by observation gets directly at facts and at the laws of facts. Philosophers, in their study of man and of the world, do not sufficiently consult language, the general language, the common language, that instinctive expression of the activity of the human mind. I interrogate our native language upon the question which now occupies me, and I find it reflecting the greatest light. It has, to express the results of the intellectual process which takes place in man, when regarded as the spectator of the universe and of himself, many different words: "connaître," "savoir," "croire," "connaissance," "science," "croyance," "foi." {294} These are not mere different names to express the same idea and the same fact, they are signs of different facts and of diverse states of the human soul. If we interrogate the languages of civilized nations, ancient or modern, we find in all of them, with more or less abundance, precision or subtlety, a similar variety of terms corresponding to a similar diversity of facts.