physical system of inference, but to that insight beneath the symbols which in our own minds we possess. It is by this insight that we can finally reach an answer to our question, What is Mr. X? [41] A good illustration of such substitution is afforded by astronomical observations of a certain double star with two components of equal brightness. After an intermission of observation the two components were inadvertently interchanged, and the substitution was not detected until the increasing discrepancy between the actual and predicted orbits was inquired into. [42] For example, we should most of us assume (hypothetically) that the dynamical quality of the world referred to in chapter V is characteristic of the whole background. Apparently it is not to be found in the pointer readings, and our only insight into it is in the feeling of “becoming” in our consciousness. “Becoming” like “reasoning” is known to us only through its occurrence in our own minds; but whereas it would be absurd to suppose that the latter extends to inorganic aggregations of atoms, the former may be (and commonly is) extended to the inorganic world, so that it is not a matter of indifference whether the progress of the inorganic world is viewed from past to future or from future to past. [43] This is obviously true of all experimental physics, and must be true of theoretical physics if it is (as it professes to be) based on experiment. [44] The solitary exception is, I believe, Dirac’s generalisation which introduces -numbers (p. 210). There is as yet no approach to a general system of inference on a non-numerical basis. [Pg 273] Chapter XIII REALITY The Real and the Concrete. One of our ancestors, taking arboreal exercise in the forest, failed to reach the bough intended and his hand closed on nothingness. The accident might well occasion philosophical reflections on the distinctions of substance and void—to say nothing of the phenomenon of gravity. However that may be, his descendants down to this day have come to be endowed with an immense respect for substance arising we know not how or why. So far as familiar experience is concerned, substance occupies the centre of the stage, rigged out with the attributes of form, colour, hardness, etc., which appeal to our several senses. Behind it is a subordinate background of space and time permeated by forces and unconcrete agencies to minister to the star performer.