On the Fourfold Root of the Principle of Sufficient...

Thus it is from the sensations of our body that we receive the data for the very first application of the causal law, and it is precisely by that application that the perception of this class of objects arises. They therefore have their essence and existence solely in virtue of the intellectual function thus coming into play, and of its exercise.

Now, as far as it is the starting-point, i.e. the mediator, for our perception of all other objects, I have called the bodily organism, in the first edition of the present work, the Immediate Object; this, however, must not be taken in a strictly literal sense. For although our bodily sensations are all apprehended directly, still this immediate apprehension does not yet make our body itself perceptible to us as an object; on the contrary, up to this point all remains subjective, that is to say, sensation. From this sensation certainly proceeds the perception of all other objects as the causes of such sensations, and these causes then present themselves to us as objects; but it is not so with the body itself, which only supplies sensations to consciousness. It is only indirectly that we know even this body objectively, i.e. as an object, by its presenting itself, like all other objects, as the recognised cause of a subjectively given effect—and precisely on this account objectively—in our Understanding, or brain (which is the same).

Now this can only take place when its own senses are acted upon by its parts: for instance, when the body is seen by the eye, or felt by the hand, &c., upon which data the brain (or understanding) forthwith constructs it as to shape and quality in space.—The immediate presence in our consciousness of representations belonging to this class, depends therefore upon the position assigned to them in the causal chain—by which all things are connected—relatively to the body (for the time being) of the Subject—by which (the Subject) all things are known.