Zeno Vendler quotes:

Experience and imagination must enter into the very constitution of our thoughts involving concrete individuals.

the full analysis of the notions of saying something and understanding what one said inevitably involves a concept which, as I will show in detail, essentially corresponds to the Cartesian idea of thought.

Esse est percipi, to be is to be perceived, said good old Berkeley; but, according to most philosophers, he was wrong. Yet, obviously, there are things for which the adage holds. Perception, trivially, to begin with. If elements of conscious awareness--pains, tickles, feelings of heat and cold, sensory qualia of colors, sounds, and the like--have any existence, it must consist in their being perceived by a subject.... This shows, of course, that such experiences are epiphenomenal, at least with respect to the physical world.

To be in the mind and to be known are the same thing: in this domain esse est percipi.