When the original Spanish edition of the present book was published in 1952, I had hoped that it could be translated into English. After all, the book was written in the United States, and in the very same place at which this preface is being written. The Beacon Press has realized this hope. I am happy to express to the Press, and in particular to its Director, Mr. Thomas A. Bledsoe, my sincere gratitude. I also wish to extend my acknowledgments to Mr. Willard R. Trask, who is too well known as a translator to need my praise; and to President Katharine E. McBride, of Bryn Mawr College, who has helped in various ways to make the present edition possible.

The fact that the book was written in the United States is not irrelevant for the understanding of its contents. As early as 1945, I had planned this work. Much travel and research had paved the way for writing it, but it is my experience of American life superimposed upon my European background that has given the book its specific character: its "optimistic realism," or perhaps its "realistic optimism." These expressions sound commonsensical enough to be frowned upon by both incorrigible dreamers and sophisticated prophets of doom. But it is not my fault if common sense is often truer to life than apocalyptic fantasy. It is my contention throughout the book that threats to mankind are more serious today than they ever have been, for they include the threat of self-extermination. But it is also my contention that hopes for the improvement of the whole of mankind have never been brighter than they are now. I am not willing therefore to prognosticate the ruin of mankind or to forecast for tomorrow a paradisiacal planet. Between hell and heaven there is still room for all.

The American edition contains a number of changes in respect to the original Spanish edition. Some of the changes are major ones: they include a rewriting of the first chapter, substantial alterations in a number of pages, and the addition of some long notes at the end of the last two chapters. These notes are intended to treat of problems whose importance was not commonly acknowledged at the time the book was written. I do not think that the world begins anew every day, as newspaper editors and radio and television broadcasters tend to make us believe. But I think that when major events occur, and major problems arise, they should be analyzed. Some of the changes are minor ones: they include the rephrasing of some sentences, and modifications in the bibliographical notes. This edition contains also a complete subject and name index.

Each chapter of the book is followed by its own series of notes. They are not mere footnotes which have been relegated to the end in order to free the text from them and make it more readable. They have their own character, and can be read as a continuation of the chapter. They are commentaries on the problems discussed, fuller treatment of certain interesting questions, clarifications of doubtful points. Hence they are printed in the same type as the rest of the book. Under these conditions, it has not mattered that some of the notes are rather long; the only consideration has been that they should not prove tedious.

The last note in each series is bibliographical. It provides the necessary references for the authors or books cited in the corresponding chapter. It must be borne in mind that many of the books mentioned are secondary sources. The primary sources have also been taken into account, but it has not always seemed advisable to cite them. For example, texts from the Stoics and from Plotinus have been mentioned in the notes to chapters 2 and 3, but direct references to Christian texts—the majority of which are surely familiar to the reader—have been almost entirely omitted. The case is the same with the bibliographical data for chapters 7 and 11. The author has not forgotten the principal writers of the periods he was describing, especially the philosophical writers, of whom his knowledge is least inadequate. But except when he has dealt with a specific passage from such a writer (for example, that from Locke in the last note to chapter 8), he has omitted references. Mustering texts to illustrate the various conceptions studied would not only have been cumbersome, but would have been inappropriate to the nature of this book. Thus, Descartes is discussed, and Luther, Montaigne, Machiavelli are mentioned, but no texts of theirs are cited. Moreover, some important names do not appear at all, but this does not mean that they have not been taken into consideration. Hence the bibliographical and illustrative material may appear to have been arbitrarily selected. The author asks that this be imputed not only to his ignorance, but also to his plan.