As a matter of fact, the entire conflict is more apparent than real; it centres round the meaning we wish to ascribe to the word reality, whether we conceive of reality in the pragmatic scientific sense or in the metaphysical sense as embodying “true being.” For the theoretical physicist, “reality” means that hypothesis which will permit him to co-ordinate natural phenomena with the maximum of simplicity. He knows of no other test for reality and would probably evince very little interest in the unattainable reality of the metaphysician. If, therefore, a co-ordination of the facts of experience presents greater simplicity when he assumes space to be Euclidean or non-Euclidean, then space is Euclidean or non-Euclidean in spite of the fact that phenomena might just as well have been co-ordinated (though in a less simple way) had some other hypothesis been selected. Riemann expresses the selective rôle of the criterion of simplicity when he writes: “Nevertheless it remains conceivable that the measure relations of space in the infinitely small are not in accordance with the assumptions of our geometry [Euclidean geometry], and, in fact, we should have to assume that they are not if, by doing so, we should ever be enabled to explain phenomena in a more simple way.”[14] So much for the scientific understanding of the word. But if, on the other hand, when discussing “reality” we are referring to the reality of the metaphysicians, as happens to be the case with Poincaré, then it appears quite impossible to dispute his stand; at least it is scarcely credible that any scientist would feel inclined to do so. For, assuming even that such a thing as metaphysical reality has any meaning at all, why should it be connected with simplicity of co-ordination? At any rate, Einstein himself, whose entire theory centres round a definite geometry being ascribed to the spatio-temporal background, writes: “Sub specie œternitatis, Poincaré in my opinion is right.” [Pg 56]