A thing is one in perfection, when it is complete through the presence of all that is needed for its end
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic perfection presence
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

“Therefore the Eucharist is one sacrament. I answer that, As stated in Metaph. v, a thing is said to be one, not only from being indivisible, or continuous, but also when it is complete; thus we speak of one house, and one man. A thing is one in perfection, when it is complete through the presence of all that is needed for its end; as a man is complete by having all the members required for the operation of his soul, and a house by having all the parts needful for dwelling therein. And so this sacrament is said to be one. Because it is ordained for spiritual refreshment, which is conformed to corporeal refreshment.” source