 But in the mathematical analysis of natural phenomena, Euler, along with a number of his contemporaries, did employ what amount to infinitesimals in the form of minute, but more or less concrete “elements” of continua, treating them not as atoms or monads in the strict sense—as parts of a continuum they must of necessity be divisible—but as being of sufficient minuteness to preserve their rectilinear shape under infinitesimal flow, yet allowing their volume to undergo infinitesimal change. This idea was to become fundamental in continuum mechanics.