Pragmatism or Realism?

Realism opposes a relativism of truth and upholds absolute truth. Realism says that truth is the "adequation of intellect and thing." Pragmatism says something is true insofar as it is useful. While utility might be a sign that something is true, as, e.g., the usefulness of Newtonian mechanics in inventing new technologies is a sign that it is a true explanation of the natural world, utility does not necessitate it to be true, for there might be radically different yet accurate explanations of the natural world, like quantum mechanics, which employs a completely different conceptual and philosophical framework than Newtonian mechanics.

Why must scientists return to a realist and not pragmatist definition of truth? Garrigou-Lagrange, O.P., a correspondent with the French physicist Pierre Duhem, proves that a realistic definition of truth opens one up to lines of reasoning inaccessible with a pragmatist definition of truth:

In sciences, physical and physico-mathematical, those facts which exist independently of our mind are considered certain, as laws which express constant relations among phenomena. Postulates, hypotheses, are defined by their relation to the truth to be attained, not as yet accessible or certain. To illustrate. On the principle of inertia, many scientists hold that inertia in repose is certain, meaning that a body not acted upon by an exterior cause remains in repose. But others, H. Poincare, for example, or P. Duhem, see in this view a mere postulate suggested by our experience with inertia in movement, which means that "a body already in motion, if no exterior cause acts upon it, retains indefinitely its motion, rectilinear and uniform." Experience suggests this view, because as obstacles diminish, the more is motion prolonged, and because "a constant force, acting on a material point entirely free, impresses on it a motion uniformly accelerated," as is the motion of a falling body. But the second formula of inertia, as applied to a body in repose, is not certain, because, as Poincare [La science et l'hypothese, pp. 112-19. of French original] says: "No one has ever experimented on a body screened from the influence of every force, or, if he has, how could he know that the body was thus screened?" The influence of a force may remain imperceptible.

Inertia in repose, then, remains a postulate, a proposition, that is, which is not self-evident, which cannot be proved either a priori or a posteriori, but which the scientist accepts in default of any other principle. The scientist, says P. Duhem, has no right to say that the principle is true, but neither has he the right to say it is false, since no phenomenon has so far constrained us to construct a physical theory which would exclude this principle. It is retained, so far, as guide in classifying phenomena. This line of argument renders homage to the objective notion of truth. We could not reason thus under truth's pragmatic definition.

—Reality Chapter 57: Realism And Pragmatism, III. Pragmatic Consequences

A Vatican scientist

What is metaphysics?

Metaphysics, according to the Aristotelian Thomistic meaning, is several things:

1. The science of being as being
2. The "First Philosophy" (first in the sense of "ultimate", but last in the order of our learning)
3. "Beyond physics"
4. The study of "one"

Modern philosophy, however, gives a much broader definition of metaphysics: "The branch of philosophy that deals with the first principles of things or reality, including questions about being, substance, time and space, causation, change, and identity (which are presupposed in the special sciences but do not belong to any one of them); theoretical philosophy as the ultimate science of being and knowing." (OED).

For a Thomist, "questions about" "time and space, causation, [and] change" are parts of natural philosophy, not metaphysics.

St. Thomas says of "metaphysics":

1. "one" which is convertible with being is a metaphysical entity and does not depend on matter in its being. (ST I q. 11 a. 3 ad 2)
2. …the highest of [the sciences], viz. metaphysics… (ST I q. 1 a. 8 c.)
3. …acquired knowledge about Divine things, for instance, the science of metaphysics… (ST II-II q. 9 a. 2 arg. 2)
4. …metaphysics, which treats of being or substance… (Post. Anal. I lec. 41 b)
5. Metaphysics at once studies being in general and first being, which is separated from first matter. (De generatione proem.)
6. It is called metaphysics inasmuch as it considers being and the attributes which naturally accompany being (for things which transcend the physical order are discovered by the process of analysis, as the more common are discovered after the less common). (In Meta. proem.)
7. metaphysics, which deals with divine things, is the last part of philosophy to be learned (CG I a. 4)

St. Thomas says of "physics" (natural philosophy, natural science, or philosophy of nature):

1. …physics, which treats of mobile body [i.e., changeable bodies]. (Post. Anal. I lec. 41 b)

Basically, if there are no such things as immaterial beings, physics would be the ultimate or first science (In Meta.VI lec. 1 [1170]):

if there is no substance other than those which exist in the way that natural substances do, with which the philosophy of nature deals, the philosophy of nature will be the first science. But if there is some immobile substance, this will be prior to natural substance, and therefore the philosophy which considers this kind of substance, will be first philosophy.

Also, check out the 8 tenets of River Forest / Aristotelian Thomism, which are elaborated in The Way toward Wisdom (vide the first chapter, this excerpt, John Deely's review) by Benedict Ashley, O.P., which discusses the question "What is metaphysics?"

The ultimate goal of the natural sciences is to show the existence of immaterial being(s).