Here, then, we have the difficulty, the difficulty upon which the whole of Mr. Balfour's argument depends. Solve it, and the whole sceptical argument crumbles. The naturalistic theory, we both say, is incredible. The scientific doctrines based upon it are, as we both admit, unassailable. How is this? I reply, first, because the atoms represent nothing more than a logical scaffolding which enables us to infer one set of sensible phenomena from another. We start from phenomena and we end with phenomena. When we have discovered the so-called "law"--the connecting formula--we can remove the hypothesis as the engineer can remove the provisional supports when he has once got the keystone into his arch. That this is so appears, I think, from the whole scientific procedure. How is the atomic theory obtained? Not by any direct observation of atoms themselves. They are, as Mr. Balfour says, not only not objects of observation, but incapable by their nature of ever being directly observed. The man of science begins by saying, _if_ the phenomena of light correspond in some way to a vibration of atoms, the atoms must vibrate in such and such ways. He finds, again, that the laws so discovered will give the law of other phenomena of light; and he argues quite correctly that his hypothesis is for his purpose verified. That is, it has enabled him to discover a verifiable and verified formula. In order to do this he has assumed from the very first the theory which of course appears in his conclusions. All physical science consists ultimately in giving definite formulæ in terms of space and time. It is therefore assumed that the atoms are to have no qualities except those which are definable in terms of space. We exclude any other quality because our whole purpose is to obtain purely geometrical measurements. We have asked how those atoms, infinitesimal bits, so to speak, of solid space, arranged in certain positions, must move in order to correspond to the law given by observation, and we have therefore, of course, predetermined that our answer must come out in terms of atoms.