Full Idea
Substance ...is an ens per se. No part of a substance is an ens per se when it is part of a substance, because then it would be a particular thing, and one substance would be a particular thing from many things, which does not seem to be true.
Clarification
'ens per se' (Lat) is an 'entity in itself'
Gist of Idea
Substance is an intrinsic thing, so parts of substances can't also be intrinsic things
Source
John Duns Scotus (In Praed. [1300], 15.1), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 26.1
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.607
A Reaction
The tricky bit is 'when it is a part of a substance', meaning a substance must cease to be a substance when it is subsumed into some greater substance. Maybe. Drops of water? Molecules? Bricks? Cells?
Related Idea
Idea 16775 For corpuscularians, a substance is just its integral parts [Pasnau]