Full Idea
Frege did not think of himself as working with sets.
Gist of Idea
Frege did not think of himself as working with sets
Source
report of Gottlob Frege (works [1890]) by William D. Hart - The Evolution of Logic 1
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.22
A Reaction
One can hardly blame him, given that set theory was only just being invented.