Full Idea
Armstrong takes universals generally, and structural universals along with the rest, to be abstractions from their particular instances.
Gist of Idea
Universals are abstractions from their particular instances
Source
report of David M. Armstrong (What is a Law of Nature? [1983], p.83-4) by David Lewis - Against Structural Universals 'The pictorial'
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.92
A Reaction
To me, 'abstracted' implies a process of human psychology, a way of thinking about the instances. I don't see how there can be an 'abstracted' relation which is a part of the external world. That makes his laws of nature human creations.