Dretske never met a false analogy he did not like. That pretty much wraps up my main frustration with this book. He uses SO many analogies in this book in order to explain his theory of mind, but the vast majority are so plainly not analogous to the mind.
In addition, he is operating under a definition of consciousness that Chalmers calls "The Easy Problem" and thus this book sets out to form a theory about that which needs no philosophical theory, and in doing so ignores that which is need of explanation.
And now let me quote the last paragraph of the book:
"Individuals are either conscious or they are not. It's like being pregnant. But if I am conscious and my very distant ancestors were not, when did my less-distant ancestors start being conscious? At the same time a poor man becomes rich as you keep giving him pennies."
Uhhhhhh. So why two starts? Because the representational theory and externalism about the mind (both of which he attempts to defend) are interesting theses--ones that may be better defended by others.