Philosophy and Phenomenological Research, V(1), 1-36, pdf.
Psychology has learned that perception is more than sensation.
He argues that the seeds of the Erlangen program lie in perception. When we perceive an object or one of its properties we are detecting an invariant feature of our sensations. E.g., an object of constant size although it moves; an object of constant colour, although lighting changes; a tune, although we transpose it to a different key.
“a certain type of concepts which has found its clearest expression in abstract creations of modern geometry” (p. 5)