See page 17 :
But what are concepts ? [...] Concepts are not mental presentations that intrinsically refer to external objects for the very decisive reason that they are not mental presentations at all. Concepts are signs used in a certain way; the signs may be public or private, mental entities or physical entities, but even when the signs are ‘mental’ and ‘private’, the sign itself apart from its use is not the concept.
Recall that [footnote 1] : "An older word for what I call ‘representation’ or ‘reference’ is denotation."
Thus, it seems that "mental presentation" is an image produced in the mind by an external object acting on our senses.
But we may have mental images that do not represent :
Suppose there is a planet somewhere on which human beings have evolved. Suppose these humans, although otherwise like us, have never seen trees. Suppose they have never
imagined trees. Suppose one day a picture of a tree is accidentally dropped on
their planet by a spaceship which passes on without having other contact with them. Imagine them puzzling over the picture. What in the world is this? All sorts of speculations occur to them: a building, a canopy, even an animal of some kind. But suppose they never come close to the truth.
For us the picture is a representation of a tree. For these humans the picture only represents a strange object, nature and function unknown. Suppose one of them has a mental image which is exactly like one of my mental images of a tree as a result of having seen the picture. His mental image is not a representation of a tree. It is
only a representation of the strange object (whatever it is) that the mysterious
Concepts are not mental; they are social :
the determination of reference is social and not individual.
Their meaning is determined by their use.