“De arte characteristica ad perficiendas scientias ratione nitentes in C. I. Gerhardt (ed.), Die philosophischen Schriften von Gottfried Wilhelm Leibniz (7 vols. 1875–1890) VII 125.”
quando orientur controversiae, non magis disputatione opus erit inter duos philosophus, quam inter duos computistas. Sufficiet enim calamos in manus sumere sedereque ad abacos, et sibi mutuo (accito si placet amico) dicere: calculemus
"[...] if controversies were to arise, there would be no more need of disputation between two philosophers than between two calculators. For it would suffice for them to take their pencils in their hands and to sit down at the abacus, and say to each other (and if they so wish also to a friend called to help): Let us calculate."
The famous calculemus of Leibniz appears in several places of his writing; this is the most frequently quoted; variants are found in the Preface to his New Essays on Human Understanding, and in Dissertatio de Arte Combinatoria (1666). See R. Chrisley, Artificial Intelligence (2000), p. 14 https://books.google.ch/books?id=dLQ3bDy2tgYC&pg=PA14#v=onepage&q&f=false; H. Busche, Leibniz' Weg ins perspektivische Universum (1997), p. 134 https://books.google.ch/books?id=xAI4Wtp0GBoC&pg=PA134&lpg=PA134.
