Ghosts of Departed Quantities

Concepts such as infinitesimal numbers and fluxions have been used by Leibnitz and Newton for the initial development of calculus. However, their non-rigorous nature has caused a lot of controversy and they have eventually been phased out by epsilon-delta definitions. In early 60s Abraham Robinson realised that methods of mathematical logic can be used to provide rigorous meaning to such concepts. This talk is a gentle introduction to some of Robinson's ideas.