587. If there exists a subject worthy of being studied critically on account of the frequency and variety of the cases it presents, it is unquestionably this; for whether we contemplate the works of nature or of art, their varied colours form one of the finest spectacles man is permitted to enjoy. Hence the desire of reproducing the coloured images of objects which excite our admiration or interest, has produced the art of painting. The imitation of the painter’s works has given birth to tapestry, and carpets, and mosaics; while the necessity for economically multiplying designs has led to ornamental printing. The love of colour has also induced man to paint his dwelling and to dye his garments and household decorations. 588. The sight of colours, so simple a thing for the greater part of mankind, is, according to some philosophers, a phenomenon entirely out of the domain of positive knowledge, inasmuch as they consider that it varies with the organization, and even the imagination, of individuals; consequently, they think that it cannot be inferred that, because one man sees an object in a certain way, another will see it in like manner under the same external circumstances. They believe that no generalization, deduced from observation, can direct the artist with certainty, either in the art of seeing his model or in faithfully reproducing a coloured image of it: they also think that no useful physiological generalization can arise from a profound study of the modifications his organs experience from the sight of the colours that bodies present to him. I cannot admit that we ought to abstain from the study of a subject because it presents variable phenomena. All those who are engaged in the study of the positive sciences, should inquire for some fact capable of illustrating the study of these phenomena.