Jackson's thought experiment sets this up. Mary has lived her life entirely in a black and white room, has never seen any other colors. But she's put the time to good use: she has learned every true proposition about how the brain works, how it processes dolors, even detailed descriptions of the neural processes which give rise to the conscious experienced of color.
After this, somebody at long last brings red rose into the room. Question: has Mary learned anything? Answer, no. By hypothesis, she already knows every true proposition about her present experience of the rose.
But what she does have is a non-propositional conscious experience which she's never had before. This has to be the correct answer, because of the work of another Philosopher, Wilfred Sellars. His (in)famous essay "Empiricism and the Philosophy of Mind" gives a knock-down argument that conscious experiences of things like color cannot have propositional content. They are not like sentences, they are more like rocks and trees, i.e. real objects, but they cannot be true or false, and therefore they cannot be propositions.
Alas, reading the essay "Empiricism and the Philosophy of Mind" is a journey that many start, but few finish :-( It is very definitely the hardest thing to understand that I ever read. But indeed, it does inescapably prove that proposition just can't be used to make conscious experiences.