In English, what we call imagination tends to link together what St. Thomas would have distinguished into several faculties. Everyone seems to agree that imagination is a sense power that presupposes a unified sensory experience- in other words, when we imagine a new car we can presuppose that its color, sound, smell have already been united. We can imagine a single sensation too, but our capability to imagine what is already united points to a power that has already united the various sensations of color, sound, texture, taste, movement etc. together. This unifying sense faculty is what Aristotle and St. Thomas call “the common sense”, but it is only probable that this sensation is really different from imagination.
For some Thomists, imagination seems like a rather unremarkable thing: it’s defined as an interior sense that retains the images of common sense. This definition does not make explicit the primary thing that the English speaker means by “imagination”, for we tend to mean a creative interior sensory power. Imagination also seems to connote the loftiest of the sensory powers- it is the closest to intellect and it it perfects a unified sensory perception. This loftiness and creativity of imagination accounts for both its dignity and its danger- it is a certain middle between a particular sense power and intellect and so it is easy to reduce it to both, and to reduce- more dangerously- intellect to imagination.
Just Thomism 