Simson's treatise, "De porismatibus", begins with definitions of theorem, problem, datum, porism and locus. The term originates from three books of Euclid with porism, that have been lost. Proclus points out that the word "porism" was used in two senses. A locus (says Simson) is a species of porism. A porism is a mathematical proposition or corollary.