Quotations

Émile Picard

In reality the origin of the notion of derivatives is in the vague feeling of the mobility of things, and of the greater or less speed with which phenomena take place; this is well expressed by the terms fluent and fluxion, which were used by Newton and which we may believe were borrowed from the ancient mathematician Heraclitus.

True rigour is productive, being distinguished in this from another rigour which is purely formal and tiresome, casting a shadow over the problems it touches.