Cauchy's sum theorem is a prototype of what is today a basic result on the convergence of a series of functions in undergraduate analysis. We seek to interpret Cauchy’s proof, and discuss the related epistemological questions involved in comparing distinct interpretive paradigms. Cauchy’s proof is often interpreted in the modern framework of a Weierstrassian paradigm. We analyze Cauchy’s proof closely and show that it finds closer proxies in a different modern framework.

Summary The historical development of the non-standard analysis is sketched. With the help of this mathematical branch infinite and infinitesimal quantities are placed in an extension of the real numbers and so find their justification. In this way an old mathematical and philosophical problem is solved in the 20th century, but not in such a manner, mathematicians with classical „standard methods thought of.