Nicki wrote:So - rereading your posts as well - the form is the same thing as the soul, and produces a body.
Less "the same thing as" and more the definition of. The soul, by definition, is the form of the body. So, yes, the soul is what produces the body. Think of it this way: the matter that makes up your body is informed
by the soul.
How does that fit in with DNA making the body what it is?
DNA is a part of the body and so informed (that is, made what it is) in virtue of the soul. Human DNA is human DNA precisely because it is a human soul that informs it.
And why is intellect immaterial, but not imagination?
Imagination, like the intellect, is an internal faculty of the soul (there are others as well--memory, will, etc.). The intellect is not identical with the soul, but is (like imagination) a power of it. The materiality or immateriality of something is related to its own nature and therefore how it is expressed/realized. The body is material insofar as the body is what the material production of the soul. It is just the nature of the body to be material. The intellect is immaterial insofar as it is the cognitive power of the soul. It is just the nature of the intellect to be immaterial. And so it is with the imagination. It's just immaterial by nature. It is not distinct from the soul, as if the immaterial part of you is part soul, part intellect, part imagination, etc. Rather, the immaterial part of you is just soul, and the soul has various powers and faculties. Some of those powers and faculties are and are expressed immaterially (even if they have material instrumental causes), e.g., the intellect and imagination. Some are and are expressed materially, e.g., the body.
For more on the imagination, read this: http://www.newadvent.org/cathen/07672a.htm