JJC Smart responds to people who would conflate experiences of seeing things with the actual things which are being seen in his 1959 paper "Sensations and Brain Processes". Here he's talking about the experience of seeing a yellow-green after image, and responding to objections to his theory that experiences can be equivalent to mental states.

Objection 4. The after-image is not in physical space. The brain-process is. So the after-image is not a brain-process.

Reply. This is an ignoratio elenchi. I am not arguing that the after-image is a brain-process, but that the experience of having an after-image is a brain-process. It is the experience which is reported in the introspective report. Similarly, if it is objected that the after-image is yellowy-orange but that a surgeon looking into your brain would see nothing yellowy-orange, my reply is that it is the experience of seeing yellowy-orange that is being described, and this experience is not a yellowy-orange something. So to say that a brain-process cannot be yellowy-orange is not to say that a brain-process cannot in fact be the experience of having a yellowy-orange after-image. There is, in a sense,no such thing as an after-image or a sense-datum, though there is such a thing as the experience of having an image, and this experience is described indirectly in material object language, not in phenomenal language, for there is no such thing. We describe the experience by saying, in effect, that it is like the experience we have when, for example, we really see a yellowy-orange patch on the wall. Trees and wallpaper can be green, but not the experience of seeing or imagining a tree or wallpaper. (Or if they are described as green or yellow this can only be in a derived sense.)

The theory he is defending in the paper is an identity theory where brain states are identical to mental states, but the point still holds for functionalist theories where mental states supervene on functional states.