Quantitative: countable. We have a soul. One. It's a thing. Not part of our imagination.
Immeasurable: it doesn't have weight or mass. There's nothing about a soul that science can point to, independent of the body, in order to demonstrate its presence.
It might be easier if I asked you what a soul is, independent of the body.
I'm suggesting that St. Thomas Quinas' meditations on the soul carry no more weight in the real world than George Lucas' notes on how The Force works. (If you can think of a polite way for me to say that, I'm all ears)