Appendix 29.

(810) [Judgment about the identity of a body]

Generally speaking, we can show that when we touch bodies with different parts of our own body we do not perceive their identity. Different perceptions of external things correspond to different affected parts of our body, and the bodies (agents acting on us) seem as many as the perceptions we have in different parts of our body, especially if the perceptions take place simultaneously. Nevertheless, when we are touched in (phenomenally) continuous space, touch gives notice of several bodies forming a continuum amongst themselves, as happens with solids. On the other hand, if we have non-continuous sensations, for instance, when we are touched by a body on our hand and again on our foot, we can only think that two bodies have touched us. Only the use of sight, or the continuity of touch, as I said, and habit, enables us to judge the unity of a body.

The judgment that we make about the identity of a body touching us simultaneously on several parts of our body is an habitual judgment, dependent upon experience, and as such can sometimes deceive us. For example, if I touch a button with two fingers crossed one over the other, I feel two buttons because I feel two sensations in different parts of my fingers where I am not accustomed to be touched simultaneously by a single body. The natural position of my fingers, when I touch a body, is straight out and flat so that the sensations produced by an external body are close together. On the other hand, when my fingers are crossed, one sensation takes place at a distance from the other and at a part of the tip opposite to where it normally happens.

In our case of the pencil tip running along the arm, we have the (phenomenal) continuity of a sliding sensation which makes us believe that the body touching me is the same although different parts of the arm are touched. Nevertheless, simple touch tells us that only similar sensations succeed one another without noticeable interruption. This is certainly not sufficient to prove the movement of the external body. On the contrary, when I take a body in my hand and carry it from one place to another, the identity of the body is proved by the continuity of its perception, unmoved relative to my hand grasping the body. In this case, I would perceive the movement not with simple touch, but with touch assisted by interior awareness of my arm which I move.