A great part of the present volume consists of chapters which composed the twelfth Book of the Philosophy in former editions, which Book was then described as a 'Review of Opinions on the nature of Knowledge and the Method of seeking it.' I have added to this part several new chapters, on Plato, Aristotle, the Arabian Philosophers, Francis Bacon, Mr. Mill, Mr. Mansel, the late Sir William Hamilton, and the German philosophers Kant, Fichte, Schelling and Hegel. I might, if time had allowed, have added a new chapter on Roger Bacon, founded on his _Opus Minus_ and other works, recently published for the first time under the direction of the Master of the Rolls; a valuable contribution to the history of philosophy. But the review of this work would not materially alter the estimate of Roger Bacon which I had derived from the _Opus Majus_. But besides these historical and critical surveys of the philosophy of others, I have ventured to introduce some new views of my own; namely, views which bear upon the philosophy of religion. I have done so under the conviction that no philosophy of the universe can satisfy the minds of thoughtful men which does not deal with such questions as inevitably force themselves on our notice, respecting the Author and the Object of the universe; and also under the conviction that every philosophy of the universe which has any consistency must suggest answers, at least conjectural, to such questions. No _Cosmos_ is complete from which the question of Deity is excluded; and all Cosmology has a side turned towards Theology. Though I am aware therefore how easy it is, on this subject, to give offence and to incur obloquy, I have not thought it right to abstain from following out my philosophical principles to their results in this department of speculation. The results do not differ materially from those at which many pious and thoughtful speculators have arrived in previous ages of the world; though they have here, as seems to me, something of novelty in their connection with the philosophy of science. But this point I willingly leave to the calm decision of competent judges. I have added in an Appendix various Essays, previously published at different times, which may serve perhaps to illustrate some points of the history and philosophy of science. TRINITY LODGE, _February 8, 1856_. ON THE PHILOSOPHY OF DISCOVERY. CONTENTS. The chapters marked thus * appear now for the first time. The chapters marked thus † have appeared in other works. CHAP. I. INTRODUCTION. CHAP. II. PLATO. CHAP. III. * ADDITIONAL REMARKS ON PLATO. 1. The Doctrine of Ideas. 2. The Doctrine of the One and Many. 3. The notion of the nature and aim of Science. 4. The Survey of existing Sciences. 5. The Constitution of the human Mind. CHAP. IV. ARISTOTLE.