I have numbered the above propositions, as it is important to keep them separate. On the whole, (1), (2), and (3) seem to me to be true, but (4) and (5) seem to me to be false. Behaviourists, I think, incline to the view that (4) and (5) follow from (1), (2), and (3); but this view I attribute to what I should regard as errors concerning the basis of physics. That is why it was necessary to discuss physics before coming to a decision on this question of self-observation. But let us examine each of the above propositions in turn. (1) It is true that the facts upon which the physical sciences are based are all of them public, in the sense that many men can observe them. If a phenomenon is photographed, any number of people can inspect the photograph. If a measurement is made, not only may several people be present, but others can repeat the experiment. If the result does not confirm the first observer, the supposed fact is rejected. The publicity of physical facts is always regarded as one of the greatest assets of physics. On a common-sense basis, therefore, the first of the propositions in which we have summed up the behaviourist philosophy must be admitted. There are, however, some very important provisos which must be mentioned. In the first place, a scientific observer is not expected to note his integral reaction to a situation, but only that part of it which experience leads him to regard as “objective”, _i.e._ the same as the reaction of any other competent observer. This process of learning to note only “objective” features in our reaction is, as we have seen, begun in infancy; training in science only carries it further. A “good” observer does not mention what is peculiar to himself in his reaction. He does not say: “A boring speck of light danced about, causing me eye-fatigue and irritation; finally it settled at such-and-such a point.” He says simply: “The reading was such-and-such”. All this objectivity is a result of training and experience. One may say, in fact, that very few men have the “right” reaction to a scientific situation. Therefore an immense amount of theory is mixed up with what passes in science as pure observation. The nature and justification of this theory is a matter requiring investigation.