Full Idea
A composite entity is a unum per se if the partial entities that are in it are contained under one common essence. …In water, all those parts are contained under one essence of water.
Gist of Idea
A composite is a true unity if all of its parts fall under one essence
Source
Chistoph Scheibler (Metaphysics [1650], I.4.1 n9), quoted by Robert Pasnau - Metaphysical Themes 1274-1671
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.594
A Reaction
Water mixed with wine is said to be an 'ens per accidens'. This is an unusual but possible view, that all the water there is is a single thing, united by its compositional essence. When we talk about 'water', we include possible water, and past water.