Page "Non-standard analysis" Paragraph 4

:[...] the idea of infinitely small or infinitesimal quantities seems to appeal naturally to our intuition.

At any rate, the use of infinitesimals was widespread during the formative stages of the Differential and Integral Calculus.

As for the objection [...] that the distance between two distinct real numbers cannot be infinitely small, G. W. Leibniz argued that the theory of infinitesimals implies the introduction of ideal numbers which might be infinitely small or infinitely large compared with the real numbers but which were to possess the same properties as the latter.