Introduction

To many of the early Greek philosophers it seemed that only those things which are changeless could be made the subject of scientific study. How, they asked, is it possible to have any knowledge about something which is in process of becoming something else? The very notion of change seems unreal, for how can one thing cease to exist and become another?

This problem continues to give trouble, although in a less acute form, and the atomic theory provided what is at least a partial answer. According to this theory the occurrence of a natural process is regarded as being simply a change in the mutual positions of the atomic particles. The latter are thought of as being entities which are permanent and changeless, and it is this postulate which makes it possible to put forward an idea of change, i.e. in terms of changes of pattern, which is at least comprehensible.

The modern theory of rates is therefore based, in the first place, on the particular types of particles which may be assumed to remain unchanged in the process which is under discussion, e.g. the atoms in chemical reactions. However, this way of looking at things serves only to diminish the difficulties and not to eliminate them. There is no theory of rates which stands, so to speak, on its own feet; all existing theories depend, in one form or another, on ideas carried over from the study of matter at equilibrium, which is to say in an unchanging condition.