Appreciation of sculpture depends upon the ability to respond to form in three dimensions. That is perhaps why sculpture has been described as the most difficult of all arts; certainly it is more difficult than the arts which involve appreciation of flat forms, shape in only two dimensions. Many more people are 'form-blind' than colour-blind. The child learning to see, first distinguishes only two - dimensional shape; it cannot judge distances, depths. Later, for its personal safety and practical needs, it has to develop (partly by means of touch) the ability to judge roughly three - dimensional distances. But having satisfied the requirements of practical necessity, most people go no further. Though they may attain considerable accuracy in the perception of flat form, they do not make the further intellectual and emotional effort needed to comprehend form in its full spatial existence.
This is what the sculptor must do. He must strive continually to think of, and use, form in its full spatial completeness. He gets the solid shape, as it were, inside his head - he thinks of it, whatever its size, as if he were holding it completely enclosed in the hollow of his hand. He mentally visualizes a complex form from all round itself; he knows while he looks at one side what the other side is like; he identifies himself with its centre of gravity, its mass, its weight; he realizes its volume, as the space that the shape displaces in the air.
And the sensitive observer of sculpture must also learn to feel shape simply as shape, not as description or reminiscence. He must, for example, perceive an egg as a simple single solid shape, quite apart from its significance as food, or from the literary idea that it will become a bird. And so with solids such as a shell, a nut, a plum, a pear, a tadpole, a mushroom, a mountain peak, a kidney, a carrot, a tree - trunk, a bird's beak, a cloud, a flower. From these he can go on to appreciate more complex forms of combinations of several forms.
The story of the discovery of Troy by Heinrich Schliemann is a good example of the fact that the recognition of the form of an object may be quite independent of its significance. Schliemann, as a child, had heard the story of the destruction of Troy from his father. He became obsessed with the idea that he would find the site of ancient Troy. Years later, when he had made a fortune as a merchant in Russia, he set out on his quest. Guided by the description in Homer's Iliad, he dug at a site in modern Turkey which he thought was the location of Troy. After many years of fruitless digging, he finally found the ruins of a city which he believed was Troy. But what he had really found was a city which had been built on the site of Troy long after the Trojan War. The significance of the discovery was that it showed that the form of the city could be recognized even when its significance had been completely forgotten.
Another example of the independence of form is to be found in the work of the American sculptor Alexander Calder. Calder was famous for his mobiles, which are sculptures that move in the air. Calder's mobiles are made up of simple geometric shapes such as circles, squares, triangles, and rods. These shapes are joined together by wires or rods so that they can move freely in the air. The significance of Calder's mobiles is not in their description of anything in particular, but in their form. They are a celebration of form in movement, of the way in which simple geometric shapes can be combined to create complex and beautiful forms in motion.