PITHY QUOTE FROM Religious Studies
But just as optics is subalternated to geometry, so the science of the rainbow is subalternated to optics, for it applies to a determinate matter the principles which optics hands down absolutely. Hence it belongs to the naturalist who treats of the rainbow to know the quia, but to the expert in optics to know the propter quid. For the naturalist says that the cause of the rainbow is the convergence of a visual line at a cloud arranged in some relation to the sun; but the propter quid he takes from optics.
The Collected Works of St. Thomas Aquinas. Commentary on the Posterior Analytics of Aristotle, Book I, Lecture 25