Johann Heinrich Lambert (1728–1777), German physicist, mathematician, and astronomer, devoted a number of essays to the enterprise of making a calculus of logic, which he evidently thought of in connection with the tree of Porphyry. His standpoint is, as is usual with the early investigators, intensional. Let a and b be any concepts, a + b their combination into a compound concept, and ab their common part. The letters γ and δ can be multiplied with conceptual variables, so that aγ and aδ are read as "the genus of a" and "the difference of a." The intended meaning suggests that γ and δ are descriptive operators; yet Lambert sometimes treated them as though they were placeholders for generic or differential concepts. At any rate Lambert, following an elementary intuition, posited a = aγ + aδ = a(γ + δ). Wanting to descend the tree to subordinate species as well as to ascend to superordinate genera...