This book is basically a translation of a work which I originally wrote in Spanish, and which was published in 1962. It is not, however, a mere duplicate, in another tongue, of the original version. It differs from the latter in various important respects. To begin with, I have revised the text throughout in order to make it more concise without loss of meaning. I have added a number of paragraphs on some crucial points which I felt needed clarification. I have entirely rewritten half a dozen sections, and in many cases I have changed the order of presentation.

As a consequence, I hope that, while still fundamentally a translation, the book has been considerably improved. Indeed, if a new Spanish edition of the book is produced, I will probably rewrite large portions of it in accordance with the present English text.

I wish to express my thanks to Mr. Miguel Gonzalez-Gerth for his help in the translation. I also wish to express to the University of California Press, and in particular to its director, Mr. August Fruge, my sincere appreciation for issuing a book which, although on a topic of very general interest, remains a philosophic work, and makes no pretense of making difficult things easy. I should add that the book makes no pretense of making easy things difficult. From the point of view of its possible appeal to the public, making easy things difficult may be occasionally a more effective procedure than making difficult things easy or simply letting things be what they are. I know of not a few cases of works whose authors have succeeded in making easy things very arduous, and have subsequently enjoyed a wide reputation. To be sure, few people have read such works, but fewer still have dared confess that they did not. I have myself nothing against success—indeed, I sincerely hope that the present book will attain some mesaure of it—but I feel that, if it comes, it should be the result of understanding rather than of misunderstanding. Since in the world in which we live we have already had a good share of the latter, it may not seem too unreasonable to claim some of the former.