Full Idea
Like psychological abstractionism, Frege's method (which we can call 'logical abstraction') aims at isolating what is in common between the members of any equivalent sets of objects, by identifying the feature with the maximal set of such objects.
Gist of Idea
Frege's logical abstaction identifies a common feature as the maximal set of equivalent objects
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.14
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.167
A Reaction
[compressed] So Frege's approach to abstraction is a branch of the view that properties are sets. This view, in addition to being vulnerable to Russell's paradox, ignores the causal role of properties, making them all categorical (which is daft).