The definitions of physics proceed according to the method immortalised in “The House that Jack built”: This is the potential, that was derived from the interval, that was measured by the scale, that was made from the matter, that embodied the stress, that.... But instead of finishing with Jack, whom of course every youngster must know without need for an introduction, we make a circuit back to the beginning of the rhyme: ... that worried the cat, that killed the rat, that ate the malt, that lay in the house, that was built by the priest all shaven and shorn, that married the man.... Now we can go round and round for ever. But perhaps you have already cut short my explanation of gravitation. When we reached matter you had had enough of it. “Please do not explain any more, I happen to know what matter is.” Very well; matter is something that Mr. X knows. Let us see how it goes: This is the potential that was derived from the interval that was measured by the scale that was made from the matter that Mr. X knows. Next question, What is Mr. X? Well, it happens that physics is not at all anxious to pursue the [Pg 263] question, What is Mr. X? It is not disposed to admit that its elaborate structure of a physical universe is “The House that Mr. X built”. Fig. 8 It looks upon Mr. X—and more particularly the part of Mr. X that knows—as a rather troublesome tenant who at a late stage of the world’s history has come to inhabit a structure which inorganic Nature has by slow evolutionary progress contrived to build. And so it turns aside from the avenue leading to Mr. X—and beyond—and closes up its cycle leaving him out in the cold. From its own point of view physics is entirely justified. That matter in some indirect way comes within the purview of Mr. X’s mind is not a fact of any utility for a theoretical scheme of physics. We cannot [Pg 264] embody it in a differential equation. It is ignored; and the physical properties of matter and other entities are expressed by their linkages in the cycle. And you can see how by the ingenious device of the cycle physics secures for itself a self-contained domain for study with no loose ends projecting into the unknown. All other physical definitions have the same kind of interlocking. Electric force is defined as something which causes motion of an electric charge; an electric charge is something which exerts electric force. So that an electric charge is something that exerts something that produces motion of something that exerts something that produces ... ad infinitum.