But by imitation in art, Leonardo intended no slavish reproduction of nature. When he wrote that "the painter strives and competes with nature," he was on the track of a more Aristotelian idea. This he barely developed, using nature only partly in the Stagirite's sense, of inner force outwardly exemplified. The idea of imitation, in fad, as it presented itself to his mind, was two-fold. It was not merely the external reproduction of the image, which was easy enough to secure. The real difficulty of the artist lay in reflecting inner character and personality. It was Leonardo's firm conviction that each thought had some outward expression by which the trained observer was able to recognize it. Every man, he wrote, has as many movements of the body as of varieties of ideas. Thought, moreover, expressed itself outwardly in proportion to its power over the individual and his time of life. By thus employing bodily gesture to represent feeling and idea, the painter could affect the spectator whom he {xv} placed in the presence of visible emotion. He maintained that art was of slight use unless able to show what its subject had in mind. Painting should aim, therefore, to reproduce the inner mental state by the attitude assumed. This was, in other words, a natural symbolism, in which the symbol was no mere convention, but the actual outward projection of the inner condition of the mind. Art here offered an equation of inward purpose and outward expression, neither complete without the other. Further than this, influenced by Platonic thought, Leonardo's conception of painting was, as an intellectual state or condition, outwardly projected. The painter who practised his art without reasoning of its nature was like a mirror unconsciously reflecting what was before it. Although without a "manual act" painting could not be realized, its true problems--problems of light, of colour, pose and composition, of primitive and derivative shadow--had all to be grasped by the mind without bodily labour. Beyond this, the scientific foundation in art came through making it rest upon an accurate knowledge of nature. Even experience was only a step towards attaining this. "There is nothing in all nature without its reason," he wrote. "If you know the reason, you do not need the experience."