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