But is not this critical review of the whole of human knowledge an enterprise above the powers of a man?--Happily positive philosophy itself furnishes a means of lightening the task. It establishes an order which allows us to determine without too much trouble to what degree of positiveness the conception of a given category of phenomena has reached up to the present time. Comte calls this order the classification, or, more precisely the “positive hierarchy” of the fundamental sciences. It is “the plan which he will follow in the exposition of positive philosophy.”[23] This plan is not a simple artifice destined to make the entirety of the doctrine clearer, or its exposition easier. It is not external to the work. It is born from the very spirit of positive philosophy; it expresses the spirit of that philosophy in a new form. It is the natural complement of the law of the three States. Comte puts it in plain words: “The different branches of our knowledge have not been able with equal rapidity to pass through the three great phases in their development, nor consequently to reach simultaneously the positive state. There exists, in this respect, an invariable and necessary order, which our different kinds of conceptions have followed and have been obliged to follow in their progress, and of which the exact consideration is the _indispensable complement_ to the fundamental law previously enounced.”[24] Comte did not, like his contemporary Ampère, set himself the logical problem of the classification of the sciences in their entirety. He did not seek according to what principle we could arrange them all in an order where the fact of their respective subordination would be maintained. He even doubts how far such a principle exists, and he is so far from thinking of establishing a complete classification of the sciences, that he begins by leaving out the greater number of them. He first sets aside all forms of human knowledge which refer to art, that is to say all the applied sciences, practical and technical. Similarly he sets aside all the concrete sciences, such as zoology, mineralogy, geography, etc. He only places within his classification the _theoretical and abstract_ sciences, that is to say those which have no other object but the knowledge of laws, and which study phenomena, exclusive of the concrete beings in which these phenomena present themselves. Comte calls them “fundamental” because the other sciences suppose their existence, whereas the abstract sciences do not suppose the existence of the others before them.