Extracted from Harold Speed’s The Practice and Science Of Drawing:
The fact that we have two flat pictures on our two retinas to help us, and that we can focus at different planes, would not suffice to account for our knowledge of the solidity and shape of the objective world, were these senses not associated with another sense all important in ideas of form, the sense of touch.
This sense is very highly developed in us, and the earlier period of our existence is largely given over to feeling for the objective world outside ourselves. Who has not watched the little baby hands feeling for everything within reach, and without its reach, for the matter of that; for the infant has no knowledge yet of what is and what is not within its reach. Who has not offered some bright object to a young child and watched its clumsy attempts to feel for it, almost as clumsy at first as if it were blind, as it has not yet learned to focus distances. And when he has at last got hold of it, how eagerly he feels it all over, looking intently at it all the time; thus learning early to associate the “feel of an object” with its appearance. In this way by degrees he acquires those ideas of roughness and smoothness, hardness and softness, solidity, &c., which later on he will be able to distinguish by vision alone, and without touching the object.
Our survival depends so much on this sense of touch, that it is of the first importance to us. We must know whether the ground is hard enough for us to walk on, or whether there is a hole in front of us; and masses of colour rays striking the retina, which is what vision amounts to, will not of themselves tell us. But associated with the knowledge accumulated in our early years, by connecting touch with sight, we do know when certain combinations of colour rays strike the eye that there is a road for us to walk on, and that when certain other combinations occur there is a hole in front of us, or the edge of a precipice.
And likewise with hardness and softness, the child who strikes his head against the bed-post is forcibly reminded by nature that such things are to be avoided, and feeling that it is hard and that hardness has a certain look, it avoids that kind of thing in the future. And when it strikes its head against the pillow, it learns the nature of softness, and associating this sensation with the appearance of the pillow, knows in future that when softness is observed it need not be avoided as hardness must be.
Sight is therefore not a matter of the eye alone. A whole train of associations connected with the objective world is set going in the mind when rays of light strike the retina refracted from objects. And these associations vary enormously in quantity and value with different individuals; but the one we are here chiefly concerned with is this universal one of touch. Everybody “sees” the shape of an object, and “sees” whether it “looks” hard or soft, &c. Sees, in other words, the “feel” of it.
The drawing in natural red chalk by Alfred Stephens.