The collection of technical knowledge at the service of artists desirous of externalizing their expressions, can be divided into groups, which may be entitled _theories of the arts_. Thus is born a theory of Architecture, comprising mechanical laws, information relating to the weight or to the resistance of the materials of construction or of fortification, manuals relating to the method of mixing chalk or stucco; a theory of Sculpture, containing advice as to the instruments to be used for sculpturing the various sorts of stone, for obtaining a successful fusion of bronze, for working with the chisel, for the exact copying of the model in chalk or plaster, for keeping chalk damp; a theory of Painting, on the various techniques of tempera, of oil-painting, of water-colour, of pastel, on the proportions of the human body, on the laws of perspective; a theory of Oratory, with precepts as to the method of producing, of exercising and of strengthening the voice, of mimic and gesture; a theory of Music, on the combinations and fusions of tones and sounds; and so on. Such collections of precepts abound in all literatures. And since it soon becomes impossible to say what is useful and what useless to know, books of this sort become very often a sort of encyclopaedias or catalogues of desiderata. Vitruvius, in his treatise on Architecture, claims for the architect a knowledge of letters, of drawing, of geometry, of arithmetic, of optic, of history, of natural and moral philosophy, of jurisprudence, of medicine, of astrology, of music, and so on. Everything is worth knowing: learn the art and lay it aside. It should be evident that such empirical collections are not reducible to a science. They are composed of notions, taken from various sciences and teachings, and their philosophical and scientific principles are to be found in them. To undertake the construction of a scientific theory of the different arts, would be to wish to reduce to the single and homogeneous what is by nature multiple and heterogeneous; to wish to destroy the existence as a collection of what was put together precisely to form a collection. Were we to give a scientific form to the manuals of the architect, the painter, or the musician, it is clear that nothing would remain in our hands but the general principles of Mechanic, Optic, or Acoustic. Or if the especially artistic observations disseminated through it be extracted and isolated, and a science be made of them, then the sphere of the individual art is deserted and that of Aesthetic entered upon, for Aesthetic is always general Aesthetic, or better, it cannot be divided into general and special. This last case (that is, the attempt to furnish a technique of Aesthetic) is found, when men possessing strong scientific instincts and a natural tendency to philosophy, set themselves to work to produce such theories and technical manuals. [Sidenote] _Critique of the aesthetic theories of the individual arts._