Human consciousness, then, in the only form in which it can be examined and described, is a compound of various elements, of whose separate action, if it ever existed, we retain no remembrance, and therefore no power of reproducing ie. thought. It is impossible to have a distinct conception of an act of pure sensation— ie. of an affection of the organs of sense only, unaccompanied by reflection upon it; for such an affection, though possibly the earliest step in our mental development, could not at that time be recognised as such, nor leave traces that could be recognised afterwards.
Our personal consciousness, like the air we breathe, comes to us as a compound; and we can no more be conscious of the actual presence of its several elements than we can inhale an atmosphere of pure azote. Hence it follows that in distinguishing and describing the several phenomena of consciousness, we must describe them according to their predominant characteristics as compounds, not according to their separate natures as simples.
The phenomena, for example, of sensation, are so called from their prominent feature; the presence, that is to say, of an object affecting in a certain way the organs of sense; though the consciousness of the manner of that affection in each case, and consequently its existence as a distinct phenomenon, depend likewise upon the co-operation of other faculties, which play a necessary though a subordinate part.
***Excerpt from Henry Longueville Mansel, B.d.. Metaphysics or the philosophy of consciousness