Hobbes on imagination: mental images or ideas?

Question

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)?

Answer 1

As @MauroAllegranza stated in a comment, Hobbes has been materialistic and nominalistic.

Taking this, the answer of @JoWehler is not completely correct: Of course, a mathematical (infinite) line can be seen as a "compound imagination". A composition of several finite lines or even one and the same finite line. But Hobbes point would be that, in thinking (imagining) a mathematical, one-dimensional line, we have to refer to our compound imagination of a (not strictly one-dimensional or infinite) line.

His point is bold: If we want to imagine a line, not only a symbol for it like the vector (2,5), we have to imagine something visual. We cannot possibly imagine anything one-dimensional or infinite. Thoughts like the one you mention are of another kind. They are neither memories nor imaginations of anything. They are thoughts like this imagination, but one-dimensional and infinite, reducing (or extending) an image to something we have no image of, an abstraction of an imagination.

In this sense, memories and imaginations are one and the same: They are real content of conciousness. Not only any abstraction/idea/momentum/proposition. Hobbes in this sense would have rejected the idea of the reality of abstract/ideal entities.

Answer 2

The quote of your question is from chapter 2 of Hobbes, Thomas: Leviathan.

Hobbes explains how our sensual impressions fade away when we experience new impressions: This Decaying Sense can be considered from two different points of view: We name Imagination its content. And we name Memory its property of fading away.

So that imagination and memory are but one thing, which for diverse considerations hath diverse names.

Hobbes seems a bit unprecise. In the first part of his statement he equalizes imagination with memory, while in the second he correctly separates both as two aspects of the same object. Nevertheless one can understand what he means.

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. [...]

Hobbes speaks about imaginations which derive from former sense perceptions. Either one imagination corresponds to one sense perception (simple imagination). He continues: Or one imagination corresponds to several sense perceptions, which in combination make up the imagination (compound imagination).

At this point of his investigation Hobbes does not consider an imagination which does not correspond to a previous sense perception alike to your example of the line without width.