‘Chaos’, in the sense that concerns us, is essentially a mathematical concept, and ‘chaos theory’ is a mathematical theory. Fully to grasp the concept or understand the theory means grappling with the relevant mathematics. A few basic ideas are explained in the course of this book, alongside the more conceptual or methodological discussions.

Relatively little background mathematics is presupposed in the main text. However, at various points it is illuminating (or perhaps simply fun) to go just a little further into the details in a way that may presuppose rather more – though familiarity with some calculus and the general idea of a differential equation, plus the ability to follow a moderately abstract mathematical argument, should largely suffice. These more taxing episodes are set in sans serif type and can be skimmed or skipped by the reader.

The same typographical device also serves to mark off other episodes that in a similar way pursue philosophical details rather further than some might want to follow. By picking and choosing among these passages, readers with various interests should be able to find a path through the book to suit.

To avoid further complicating the text, references to the mathematical and philosophical literature are kept to a minimum in the main body of the book; for a modest crop of additional references, see the final section, ‘Notes’.