Sensation is a stable factor in life, independent of our will or whim, and giving us contact with outside reality…if we seriously doubt the evidence of our senses, men look strangely at us and segregate us from our fellows. Here, if anywhere, the consensus gentium is unambiguous: all men agree that if we perceive by our senses a red apple on the tree in the orchard, there really is a red apple on the tree in the orchard.
That my sensations are facts of experience there can be no doubt; all thinking has to start with the immediate facts of consciousness as given; and to deny that I perceive the various qualities of color, sound, and the rest is to deny that I have experience and that there is anything to think about…If, then, we are to be reasonable, we must start with the given facts, among which the facts of sense are obvious, solid and indubitable. Nothing else could be the criterion of the existence of a sensation than the sensation itself; if you perceive the sense datum, no argument could increase or diminish the certainty of your perception; and if you do not perceive it; not argument could call it into being. It stands or falls in its own right; is it not then an adequate criterion of truth?
Doubtless our experience of the sensation is our only criterion of the fact that we experience it; but it is doubtful whether sensations can be trusted to tell us the truth about anything more than the obvious and barren fact that they are experienced…
It is, however, significant that none of these thinkers (sensationalist and positivist school) has held with rigid consistency to the sensationalistic principle; into the thought of all of them has crept at some point a recognition of objects that it is impossible to perceive by the senses, such, for example, as consciousness…
If sensation alone be the test of truth, every sensation would, of course, be equally valid, for each sensation would be the test of its own truth. But we all believe that our senses sometimes err, and that occasionally things are not as they seem. The red apple on the tree in the orchard is doubtless there, but the oasis seen in a marriage is not…The sensations of the color-blind are supposed to be misleading. Illusions and hallucinations of sense are recognized by all psychologists. Ancient skepticism early directed attention to the untrustworthiness of the senses.
How, then, am I to be sure that a state of consciousness is a valid sensation, and not a misleading and merely subjective imagination or hallucination? Psychological experiments have shown that it is not always possible introspectively to distinguish between sensation and imagination.
The fact that we ourselves or other persons have sensations cannot itself be verified by sensation. There is no sense organ or function whereby I can have a sensation of my own sensation of blue. How much less can I have a sensation of the sensations going on in the minds of others? If, then, I believe that such sensations exist, it is because I have used some criterion beyond my immediate sensations themselves.
There are certain other facts besides sensation itself that cannot reasonably be denied, but that can never be verified by sense perception. A few illustrations will suffice. We certainly believe that real things, such as houses, streets, automobiles, exist in some sense whether we are perceiving them or not. The existence of the object is something more than our perception of it; existence itself, then, can never be perceived by the senses. Further, the mind is so constructed that it constantly uses universals; we speak of all triangles, all falling bodies; all space; without universals, scientific law would tumble into a mass of ruins…Further, self-consciousness, an every-day experience, is incapable of being tested by sensation; for the experience of being a self is an inner fact that could never be reached by any sense organ, and no serious thinker has ever pretended that he had a sensation of self-hood.
Finally we may mention ideals and values of whatever sort. If an ideal has any truth, that truth could never be tested by sensation; for the very nature of an ideal is to judge and control our life of sensation rather than to be judged or controlled by it.
(Brightman, An Introduction to Philosophy, 42-46)