s, Condillac attempted to explain how experiences of the sensation of solidity could put us in a position to infer that we have spatially extended bodies and that something else exists outside of our bodies (Treatise II.v).
These inferences arise as a consequence of the comparison of two cases, one where a being touches itself, the other where it touches something else. A being that touches itself experiences a sensation of solidity in its hand and an “answering” sensation of solidity in the part of its body that it touches, thereby experiencing itself as being solid, as being two instances of solidity, and as being two instances of solidity outside of one another. Were the being to lift its hand from its body and touch another part of its body, the pair of these sensations would cease and then after a time one member of the pair would recur in the company of a different partner. The being would experience a new sensation of solidity outside of the one it receives from its hand, but its awareness of the different body parts that it touches would be like its awareness of colours. Though experienced as being outside of one another, the touched body parts would not be experienced as forming a continuum or as spatially related to one another. However, were the being to move its hand over its body without lifting it, it would experience a continuous sequence of sensations of solidity arising from different body parts. Moving the hand repeatedly in the same way would always produce the same series of sensations. Condillac did not go into any more detail, but he seems to have presumed that this would be enough to warrant the inference that the touched body parts coexist and form a continuum. Thus, through moving its hands over itself, the “statue” is supposed to acquire an awareness that it has a spatially extended body.
When the “statue” touches another object, it experiences only one sensation of solidity without the answering sensation. The absence of the second sensation is supposed to induce the judgment that there is something else that is solid. An awareness of the shape of objects and their locations in an ambient space is then developed on this basis. The “statue” subsequently learns to attribute smells and sounds to objects by discovering that moving objects can make these sensations appear or disappear (Treatise III.i-ii). Moving its hands before its eyes and discovering how this causes colour sensations to appear or disappear leads it to think of colour sensations at first as being on the surface of its eyes, then as being outside of it beyond arm's length, and then as being on the surfaces of particular tangible objects within arms length (Treatise III.iii.1–19). Having reached this stage of development, it is ready to learn to perceive objects to be at even more remote distances through discovering associations between such visual distance cues as the clarity and apparent size of visual images and the distance that must be crossed to touch an object. The interpretation of the cues then becomes so natural that we seem to be directly seeing depth. Condillac quietly ignored his earlier objections to this position (Treatise III.iii.20–33).
Condillac's account of how sensation gives rise to the exercise of our higher cognitive faculties is broadly the same in the Essay and the Treatise. Such differences as there are between the two works arise from Condillac's decision to focus the Treatise on pre-linguistic abilities. In the Essay Condillac defined perception as the impression first occasioned in the mind by the action of objects on the senses. He maintained that we are always conscious of perceptions, but that this consciousness comes in varying degrees, which are a function of the strength with which objects act on the senses, the degree of intrinsic painfulness or pleasantness of the perception, and, most importantly, the extent to which the perception has been associated with our needs, which are ultimately a function of the pleasure and pain we receive from our perceptions. A high degree of consciousness is what 