Full Idea
Boethius argued that universals can be successfully isolated by abstraction, even if they do not exist as separate entities in the world.
Gist of Idea
If universals are not separate, we can isolate them by abstraction
Source
report of Boethius (Second Commentary on 'Isagoge' [c.517]) by Claude Panaccio - Medieval Problem of Universals 'Sources'
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.49
A Reaction
Personally I rather like this unfashionable view. I can't think of any other plausible explanation, unless it is a less conscious psychological process of labelling. Boethius's idea led to medieval 'immanent realism'.