Anyone writing a book like this one is naturally drawn into some examination of the history of the subject. Logic is different from other subjects in that it requires no apparatus or external objects of any kind whatsoever, except pencil and paper, and even these could be dispensed with by someone with a very retentive mind. All other subjects rely to some extent on external objects of some kind or other. A few poems might require no external objects, but one cannot write an ode to a daffodil without daffodils. Parts of theology might get on with no external objects but most theologies require the founder, even Old Testament Jewish theology required the physical presence of the deity. Philosophy might claim that it requires no external objects, but I am one of those who think it is largely about pseudo-problems.

This being so, the question presents itself: ‘Why weren't the main features of modern symbolic logic invented thousands of years ago?’ ‘What were the hurdles that held things up?’ One obvious hurdle was a notation of the natural numbers. The Greeks had practically nothing and the Roman system was very unsatisfactory. The Arabs however invented a satisfactory notation still used. Euclid was familiar with the axiomatic method, but said very little or nothing about rules of formation or consequence. The idea of truth was known but it doesn't seem too clear that truth is a property of sentences. Aristotle missed the relation, and by his amazing influence stifled everything for 2,000 years or more.