Logical necessity is one of the perennial problems of philosophy. Statements like ‘g = 9.81 m/sec2’ or ‘Radioactivity causes cancer’ may be physically necessary, but they are contingent: they could be false, and be refuted by new experience. By contrast, it seems that statements like ‘∼(p & ∼p)’, ‘2 + 2 = 4’, and ‘All material objects are located in space’ are necessary in a stronger sense. They do not just happen to be true, since their being false is not merely extremely improbable, but inconceivable. By the same token, disciplines like logic, mathematics and metaphysics, which seek to discover such truths, seem to be a priori, completely independent of experience.

At the turn of the century there were three accounts of this special type of truth, in particular of the ‘laws of logic’. According to psychologistic logicians like Boole, the laws of logic describe how human beings (by and large) think, their basic mental operations, and are determined by the nature of the human mind. Against this Platonists like Frege protested that logical truths are strictly necessary and objective, and that this objectivity can only be secured by assuming that their subject matter – thoughts and their structures – are not private ideas in the minds of individuals, but abstract entities inhabiting a ‘third realm’ beyond space and time. Finally, according to Russell, logical propositions are completely general truths about the most pervasive traits of reality, a view that is in some ways reminiscent of Mill's claim that mathematical propositions are well-corroborated empirical generalizations.

The nature of logical necessity preoccupied Wittgenstein from the beginning of his career, partly because he followed Russell in holding that philosophical problems are logical in nature (TLP 4.003–4.031). The early Wittgenstein took over elements of Frege's and Russell's logical systems. But his ‘philosophy of logic’, his understanding of the character of logic, and hence of philosophy, departed radically from his predecessors (TLP 4.1121, 4.126). All of the positions mentioned so far assume that logic is a science that makes statements about entities of some kind, just as empirical sciences make statements about physical objects. The ‘fundamental thought’ of the Tractatus is that this assumption is false.