In signs one observes an advantage in discovery which is greatest when they express the exact nature of a thing briefly and, as it were, picture it; then indeed the labour of thought is wonderfully diminished. — Leibniz, letter to Tschirnhaus (1678) Newton reached calculus first, but it was Leibniz who decided how it would be written. And the writing made all the difference. The letter d On 11 November 1675, in a manuscript written in Paris, Leibniz began using the letter $d$ for an infinitely small difference. The $d$ comes from the Latin differentia, from the verb differre, “to carry apart”. So: