### Ideas of Augustin-Louis Cauchy, by Theme

#### [French, 1789 - 1857, Professor at École Polytechnique.]

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###### 6. Mathematics / A. Nature of Mathematics / 5. The Infinite / k. Infinitesimals
 18085 Values that approach zero, becoming less than any quantity, are 'infinitesimals' Full Idea: When the successive absolute values of a variable decrease indefinitely in such a way as to become less than any given quantity, that variable becomes what is called an 'infinitesimal'. Such a variable has zero as its limit. From: Augustin-Louis Cauchy (Cours d'Analyse [1821], p.19), quoted by Philip Kitcher - The Nature of Mathematical Knowledge 10.4 A reaction: The creator of the important idea of the limit still talked in terms of infinitesimals. In the next generation the limit took over completely.
###### 6. Mathematics / A. Nature of Mathematics / 5. The Infinite / l. Limits
 18084 When successive variable values approach a fixed value, that is its 'limit' Full Idea: When the values successively attributed to the same variable approach indefinitely a fixed value, eventually differing from it by as little as one could wish, that fixed value is called the 'limit' of all the others. From: Augustin-Louis Cauchy (Cours d'Analyse [1821], p.19), quoted by Philip Kitcher - The Nature of Mathematical Knowledge 10.4 A reaction: This seems to be a highly significan proposal, because you can now treat that limit as a number, and adds things to it. It opens the door to Cantor's infinities. Is the 'limit' just a fiction?