Einstein’s law in its analytical form is a statement that in empty space certain quantities called potentials obey certain lengthy differential equations. We make a memorandum of the word “potential” to remind us that we must later on explain what it means. We might conceive a world in which the potentials at every moment and every place had quite arbitrary values. The actual world is not so unlimited, the potentials being restricted to those values which conform to Einstein’s equations. The next question is, What are potentials? They can be defined as quantities derived by quite simple mathematical calculations from certain fundamental quantities called intervals. (MEM. Explain “interval”.) If we know the values of the various intervals throughout the world definite rules can be given for deriving the values of the potentials. What are intervals? They are relations between pairs of events which can be measured with a scale or a clock or with both. (MEM. Explain “scale” and “clock”.) Instructions can be given for the correct use of the scale and clock so that the interval is given by a prescribed combination of their readings. What are scales and clocks? A scale is a graduated strip of matter which.... (MEM. Explain “matter”.) On second thoughts I will leave the rest of the description as “an exercise to the reader” since it would take rather a long time to enumerate all the properties and niceties of behaviour of the material standard which a physicist would accept as a perfect scale or a perfect clock. We pass on to the next question, What is matter? We have dismissed the metaphysical conception of substance. We might perhaps here describe the atomic and electrical structure of matter, but that leads to the microscopic aspects of the world, whereas we are here taking the [Pg 262] macroscopic outlook. Confining ourselves to mechanics, which is the subject in which the law of gravitation arises, matter may be defined as the embodiment of three related physical quantities, mass (or energy), momentum and stress. What are “mass”, “momentum” and “stress”? It is one of the most far-reaching achievements of Einstein’s theory that it has given an exact answer to this question. They are rather formidable looking expressions containing the potentials and their first and second derivatives with respect to the co-ordinates. What are the potentials? Why, that is just what I have been explaining to you!