The Principle of Equivalence thus asserts that \emph{some} of the chief differential equations of physics are the same for a curved region of the world as for an osculating flat region\footnotemark.\footnotetext {The correct equations for a curved world will necessarily include as a special case those already obtained for a flat world. The practical point on which we seek the guidance of the Principle of Equivalence is whether the equations already obtained for a flat world will serve as they stand or will require generalisation.} There can be no infallible rule for generalising experimental laws; but the Principle of Equivalence offers a suggestion for trial, which may be expected to succeed sometimes, and fail sometimes. The Principle of Equivalence has played a great part as a guide in the original building up of the generalised relativity theory; but now that we have reached the new view of the nature of the world it has become less necessary. Our present exposition is in the main deductive. We start with a general theory of world-structure and work down to the experimental consequences, so that our progress is from the general to the special laws, instead of vice versa. \Section{18.}{Retrospect} The investigation of the external world in physics is a quest for \emph{structure} rather than \emph{substance}. A structure can best be represented as a complex of relations and relata; and in conformity with this we endeavour to reduce the phenomena to their expressions in terms of the relations which we call intervals and the relata which we call events. If two bodies are of identical structure as regards the complex of interval relations, they will be exactly similar as regards observational properties\footnotemark,\footnotetext {At present this is limited to extensional properties (in both space and time). It will be shown later that all mechanical properties are also included. Electromagnetic properties require separate consideration.} if our fundamental hypothesis is true. By this we show that experimental measurements of lengths and duration are equivalent to measurements of the interval relation.