 While Euler treated infinitesimals as formal zeros, that is, as fixed quantities, his contemporary Jean le Rond d’Alembert (1717–83) took a different view of the matter. Following Newton’s lead, he conceived of infinitesimals or differentials in terms of the limit concept, which he formulated by the assertion that one varying quantity is the limit of another if the second can approach the other more closely than by any given quantity. D’Alembert firmly rejected the idea of infinitesimals as fixed quantities, and saw the idea of limit as supplying the methodological root of the differential calculus. For d’Alembert the language of infinitesimals or differentials was just a convenient shorthand for avoiding the cumbrousness of expression required by the use of the limit concept.