The subject of porisms's perplexed by the multitude of different views which have been held by geometers as to what a porism really was and's. The treatise which has given rise to the controversies on this subject's the Porisms of Euclid, the author of the Elements. For as much as we know of this lost treatise we're indebted to the Collection of Pappus of Alexandria, who mentions it along with other geometrical treatises, and gives a number of lemmas necessary for understanding it. Pappus state… (More on Porism)