In favour of the theory which I have briefly outlined, I do not claim that it is _certainly_ true. Apart from the likelihood of mistakes, much of it is avowedly hypothetical. What I do claim for the theory is that it _may_ be true, and that this is more than can be said for any other theory except the closely analogous theory of Leibniz. The difficulties besetting realism, the confusions obstructing any philosophical account of physics, the dilemma resulting from discrediting sense-data, which yet remain the sole source of our knowledge of the outer world--all these are avoided by the theory which I advocate. This does not prove the theory to be true, since probably many other theories might be invented which would have the same merits. But it does prove that the theory has a better chance of being true than any of its present competitors, and it suggests that what can be known with certainty is likely to be discoverable by taking our theory as a starting-point, and gradually freeing it from all such assumptions as seem irrelevant, unnecessary, or unfounded. On these grounds, I recommend it to attention as a hypothesis and a basis for further work, though not as itself a finished or adequate solution of the problem with which it deals. FOOTNOTES: [23] An address delivered to the Philosophical Society of Manchester in February, 1915. Reprinted from _The Monist_, July, 1915. [24] Cf. especially Samuel Alexander, "The Basis of Realism," _British Academy_, Vol. VI. [25] "Are Secondary Qualities Independent of Perception?" _Proc. Arist. Soc._, 1909-10, pp. 191-218. [26] First dialogue between Hylas and Philonous, _Works_ (Fraser's edition 1901). I. p. 384. [27] This point has been well urged by the American realists. [28] Cf. T.P. Nunn, "Are Secondary Qualities Independent of Perception?" _Proc. Arist. Soc._, 1909-1910. VIII THE RELATION OF SENSE-DATA TO PHYSICS I. THE PROBLEM STATED Physics is said to be an empirical science, based upon observation and experiment. It is supposed to be verifiable, i.e. capable of calculating beforehand results subsequently confirmed by observation and experiment. What can we learn by observation and experiment? Nothing, so far as physics is concerned, except immediate data of sense: certain patches of colour, sounds, tastes, smells, etc., with certain spatio-temporal relations. The supposed contents of the physical world are _prima facie_ very different from these: molecules have no colour, atoms make no noise, electrons have no taste, and corpuscles do not even smell. If such objects are to be verified, it must be solely through their relation to sense-data: they must have some kind of correlation with sense-data, and must be verifiable through their correlation _alone_.