In Modern Moral Philosophy G. E. M. Anscombe writes (page 2)
Bentham and Mill do not notice the difficulty of the concept of "pleasure".
She claimed the ancients found the concept of pleasure "baffling", but noted that
Generations of modern philosophers found this concept quite unperplexing, and it reappeared in the literature as a problematic one only a year or two ago when Ryle wrote about it.
I don't know much about Ryle, and the document I am reading does not have a bibliography, but I would like to find a reference to Ryle that addresses this difficulty of the concept of pleasure. My hope is this will help explain Anscombe's own position regarding this concept.
Anscombe, G. E. M. (1958). Modern moral philosophy. Philosophy, 33(124), 1-19. https://www.pitt.edu/~mthompso/readings/mmp.pdf
Suggested references for answers:
Ryle, G., & Gallie, W. B. (1954). Symposium: Pleasure. Proceedings of the Aristotelian Society, Supplementary Volumes, 28, 135-164. (Thanks to Eliran)
Ryle, G. Dilemmas: The Turner Lectures 1953. Cambridge University Press. (Thanks to Mauro ALLEGRANZA)
Aristotle, Nicomachean Ethics Book 2 (Thanks to virmaior)
Ryle, G. The concept of mind (1949). (Thanks to Conifold)
Katz, Leonard D., "Pleasure", The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/win2016/entries/pleasure/. (Thanks to Conifold)