We need to examine several of the new mechanical brains: Massachusetts Institute of Technology’s differential analyzer, Harvard’s IBM automatic sequence-controlled calculator, Moore School’s ENIAC (Electronic Numerical Integrator and Calculator), and Bell Laboratories’ general-purpose relay calculator. These are described in the sequence in which they were finished between the years 1942 and 1946. We also have to go on some excursions—for instance, the nature of language and of symbols, the meaning of thinking, the human brain and nervous system, the future design of machinery that can think, and a little algebra and logic. I have also sought to discuss the relations between machines that think and human society—what we can foresee as likely to happen or be needed as a result of the remarkable invention of machines that can think. READING THIS BOOK This book is intended for everyone. I have sought to put it together in such a way that any reader can select from it what he wants. Perhaps at first reading you want only the main thread of the story. Then read only what seems interesting, and skip whatever seems uninteresting. The subheadings should help to tell you what to read and what to skip. Nearly all the chapters can be read with little reference to what goes before, although some reference to the supplements in the back may at times be useful. Perhaps your memory of physics is dim, like mine. The little knowledge of physics needed is explained here and there throughout the book, and the index should tell where to find any explanation you may want. Perhaps it is a long time since you did any algebra. Then Supplement 2 on mathematics may hold something of use to you. Two sections (one in Chapter 5 and one in Chapter 6) labeled as containing some rather mathematical details may be skipped with no great loss. Perhaps you are unacquainted with logic that uses symbols—the branch of logic called mathematical logic. In fact, very few people are familiar with it. No discussion in the book hinges on understanding this subject, except for Chapter 9 where a machine that calculates logical truth is described. In all other chapters you may freely skip all references to mathematical logic. But, if you are curious about the subject and how it can be usefully applied in the field of mechanical brains, then begin with the introduction to the subject in Chapter 9, and note the suggestions in the section entitled “Algebra of Logic” in Supplement 2. In any case, glance at the table of contents, the chapter headings and subheadings, and the supplements at the back. These should give an idea of how the book is put together and how you may select what may be interesting to you.