 In SSL Lewis (1918) surveyed and discussed developments in symbolic logic from Leibniz to his own day and followed his discussion of systems of material implication with a penultimate chapter presenting his own system of strict implication (omitted from the 1960 Dover reprint). In the concluding chapter (also omitted from the 1960 Dover reprint), Lewis distinguished three possible types of “logistic procedure” and resultant system (Lewis 1918, 354–62, 367–72). The first restricts proofs in mathematics to special cases of the propositions of symbolic logic, but grants branches of mathematics their own primitive or undefined ideas and postulates distinct from those of symbolic logic. The second goes further, reducing mathematics to symbolic logic by defining or analyzing the ideas and postulates of the former in terms of those of the latter. However, the third “heterodox” or “external” conception of mathematics – “mathematics without meaning” – treats mathematical and logical systems alike as purely formal, abstract systems performing operations on recognizable ideographic marks and strings of marks in ways that are independent of what they denote or mean, or even the type of thing they are interpreted as standing for, e.g. concept or proposition, and thus independent of the truth of strings of marks. What matters instead are simply the types of order and relation with which recognizable marks or complexes of marks are distributed and “extra-logical” rules or principles for manipulating them, like the consistent substitution of marks for marks within a string of marks. Post (1921) credited Lewis as the first to make substitution an explicit rule for logic, whether under an orthodox conception as with his system of strict implication (Lewis 1918, 295) between propositions, or under a heterodox conception concerning recognizable marks and their order (Lewis 1918, 355,358), and cited Lewis’ discussion of the heterodox conception as a stimulus for his own formalist research.