I should not have presumed to choose for any lectures of mine such a subject as that which I have tried to treat in this book. The subject was chosen by the Institution where the lectures were delivered. Still less should I have presumed to print them of my own accord, knowing how fragmentary and crude they are. They were printed at the special request of my audience. Least of all, perhaps, ought I to have presumed to publish them, as I have done, at Cambridge, where any inaccuracy or sciolism (and that such defects exist in these pages, I cannot but fear) would be instantly detected, and severely censured: but nevertheless, it seemed to me that Cambridge was the fittest place in which they could see the light, because to Cambridge I mainly owe what little right method or sound thought may be found in them, or indeed, in anything which I have ever written.