I suspect that he forgot the most obvious one: I can perceive things that I have never imagined, but I cannot imagine things that I have never perceived. (I can construct images of objects that I have never seen, but those constructions are made out of objects that I have seen).

It seems to me that ultimately all these properties can be reduced to a fundamental property, the first one: percepts are passive while images are active, or, better, percepts are an act of perceiving (are caused by an interaction with the external world) while images are not an act of perceiving. This according to his descriptions.

I am not completely sure of McGinn's properties. For example, I do visualize things and people without wanting to. Every time McGinn mentions his friend Peter in the book, I visualize my friend Peter. As much as I would like to simply concentrate on the text, I can't help visualizing my friend Peter. Thus a percept may "force" an image, and the relationship between percept and image is not so obvious (in this case it is the relationship between a five-letter word and a human face).

McGinn believes that dreams are a complex system of mental images. The dream is a story told in images. He has to posit a split in the self, a dream producer versus a dream consumer, because he has posited that images are active (dreams do not look active, they look more like percepts: the way we react to dreams is exactly the way we react to percepts). His idea is that dreams are passive for the self qua consumer.

This would be the weakest part of the book if McGinn did not venture into madness, for which he thinks he has a revolutionary explanation: "madness is the imagination gone awry". Duh...

McGinn also believes that the development of logic and language was triggered by the emergence of imagination: in order to understand a sentence one has to be capable of "imagining" its content. Imagination came first, meaning came later. This sounds more convincing, although he spends precious little time on such a monumental topic.

Imagination thus becomes a fundamental cognitive faculty, the one on which the most sophisticated cognitive faculties depend. The ability to create and manipulate mental images is, in a sense, what gives us an inner life.

In my humble opinion, one big problem with mental images is that we call them "images". In reality, when I recall a friend, I also recall his voice, and, when I recall an ice cream, I also recall its taste. I even recall my dislike for spiders when I recall a spider. Thus "mental images" are actually not images at all. They are indeed something very different from visual percepts because... they are not "visual".