Thomas writes that “to signify something by words or merely by the construction of images . . . yields nothing but the literal sense” and “poetic images refer to something other than themselves only so as to signify them; and so a signification of that sort goes no way beyond the manner in which the literal sense signifies.”

If I understand this, I’m not convinced.  First, because this seems to conflate meaning with signification, which I take to be equivalent to reference.  They aren’t the same.  Second, because how we say what we say is as important as the reference of what we say.  I can refer to the same person as a “man” and as a “dirty rat”; they are both literally signifying the same person, but the way they signify inflects the reference.  ”Dirty rat” is not just a pointer, but an implicit metaphor that attributes some sort of “ratness” to the dirty rat in question.