We have continually been speaking of "Content" (although often with some hesitation) and we have discussed the possibility of two separate minds experiencing "the same" content. It is usually admitted — on the strength of arguments like those presented in section 8 — that it is forever impossible to find out whether or not two people have the same "data of consciousness" in their minds; at the same time it is generally believed that two data in different minds must either be alike or not alike, and that the question concerning their sameness has a definite meaning, although, unfortunately, it cannot be answered with absolute certainty. Usually it is added that only a high degree of probability is attainable for the answer, because sameness or diversity of mental states of different individuals 'cannot be observed directly but must be inferred by analogy'.
What are we to think of these current opinions? They seem to me to by very ambiguously expressed, and it is necessary to become perfectly clear about the meaning which the phrase "sameness of quality" can possibly have in these assertions. I think it is perfectly legimitate to say that two individuals experience "the same" or "different" feelings or qualities of sensation as long as the truth or falsity of such statements can actually be tested. Such tests are carried out by the physiologist who can examine and compare the perceptive powers of different individuals. He discovers, for instance, that most people exhibit a difference in their responses (e.g. their verbal utterances) when confronted with two different shades of colour, but that a certain percentage of individuals cannot be made to react differently in the two cases. These latter persons are called "colour blind" by the physiologist; he says that the quality of their sense perception is not the same as that of people with normal eyesight. He is perfectly right in maintaining this, and his statement is by no means based on an inference by analogy, it is an empirical judgment of the same kind of validity as any proposition in chemistry or physics. It asserts the existence of certain structures in the personality of the individuals in question: there is a difference in the multiplicity of reactions between a colour blind person and a normal one, there is a greater variety in the perceptions of a normal individual, and this is, of course, a purely formal property. This is all that can be said, and nothing else is said by the proposition 'the qualities of the sensations in the two cases differ in such and such a way'. The system
