Full Idea
Collingwood suggests art should be thought of not as product or artifact but as an act or process of expression through which the artist clarifies her initially vague emotions and states of mind. As such, it is a source of self-knowledge.
Gist of Idea
Art clarifies the artist's mind and feelings, thus leading to self-knowledge
Source
report of R.G. Collingwood (The Principles of Art [1938], Ch.6) by Stephen Davies - The Philosophy of Art (2nd ed) 8.4
Book Reference
Davies,Stephen: 'The Philosophy of Art (2nd ed)' [Wiley Blackwell 2016], p.207
A Reaction
I might believe this of writing novels, but not much else.