What is accidental to a thing does not diversify its essence.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic essence
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/19950/pg19950-images.html

Context

“or through there being but a small supply of water, or through feebleness of the minister, who cannot hold up the candidate for Baptism; or through feebleness of the candidate, whose life might be endangered by immersion. We must therefore conclude that immersion is not necessary for Baptism. Reply Obj. 1: What is accidental to a thing does not diversify its essence. Now bodily washing with water is essential to Baptism: wherefore Baptism is called a "laver," according to Eph. 5:26: "Cleansing it by the laver of water in the word of life." But that the washing be done this or that way, is accidental to Baptism.” source