Full Idea
Among the scholastics (after Duns Scotus) it would be come to be taken for granted that the crucial test for being a genuine thing - a 'res' - is separability.
Gist of Idea
Scholastics say there is a genuine thing if it is 'separable'
Source
Robert Pasnau (Metaphysical Themes 1274-1671 [2011], 11.2)
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.208
A Reaction
The idea of separability is implicit in Aristotle. In borderline cases, it seems that they are tempted to claim that things like accidental properties are separable, simply because they want them to be genuine things. A criterion for separability?