Full Idea
Whenever we have an equivalence relation among things - such as similarity in a certain respect - we can abstract under the equivalence and consider the abstractum.
Gist of Idea
Any equivalence relation among similar things allows the creation of an abstractum
Source
Peter Simons (Modes of Extension: comment on Fine [2008], p.19)
Book Reference
'Being: Developments in Contemporary Metaphysics', ed/tr. Le Poidevin,R [CUP 2008], p.19
A Reaction
This strikes me as dressing up old-fashioned psychological abstractionism in the respectable clothing of Fregean equivalences (such as 'directions'). We can actually do what Simons wants without the precision of partitioned equivalence classes.
Related Idea
Idea 18884 Abstraction is usually seen as producing universals and numbers, but it can do more [Simons]