Nature, properly speaking, does not begin to exist: rather is it the person that begins to exist in some nature.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic beginning
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

“No movement or change is denominated from the subject moved, but from the terminus of the movement, whence the subject has its species. For this reason nativity is not denominated from the person born, but from nature, which is the terminus of nativity. Reply Obj. 3: Nature, properly speaking, does not begin to exist: rather is it the person that begins to exist in some nature. Because, as stated above, nature designates that by which something is; whereas person designates something as having subsistent being. _______________________ SECOND ARTICLE [III, Q. 35, Art. 2] Whether a Temporal Nativity Should Be Attributed to Christ?” source