For the reader looking for a way into the thought of Alfred North Whitehead, this short volume is the best place to begin. In six lectures delivered toward the end of his career, Whitehead provides a non-technical sketch of the metaphysics and cosmology he had earlier presented in extended and highly technical form in his magnum opus, Process and Reality.
Modes of Thought is not an easy book--for it is highly compressed and sometimes reads like a series of aphorisms. But while this book will likely leave most readers wondering how all these aphorisms hold together, they are individually nearly crystaline in clarity and are wonderfully provocative. Even if one never reads further in Whitehead, engaging this short volume will set one pondering productively. And, if nothing else, one will come away armed with some wonderful philosophical one-liners.
If reading Modes of Thought makes one want to read on, the good way to proceed would be to read Science and the Modern World next followed by Adventures of Ideas and then (and only then) Process and Reality.