Hobbes, in his book Leviathan, second chapter, says:
So that imagination and memory are but one thing, which for diverse considerations hath diverse names. Much memory, or memory of many things, is called experience. Again, imagination being only of those things which have been formerly perceived by sense, either all at once, or by parts at several times; the former (which is the imagining the whole object, as it was presented to the sense) is simple imagination, as when one imagineth a man, or horse, which he hath seen before.
Meaning that you can't imagine what you haven't, either as a whole or as parts, experienced.
But does Hobbes talk about mental images (like the image of a horse, which you can't imagine if you've never seen a horse), or more or less "ideas" (like the "idea" of a [mathematical] line, which anyone can "imagine" but since it has only one dimension, none can visualize)?