I was reading Fr. Benedict Ashley’s The Way Toward Wisdom earlier and he makes the comment that he disagrees with Fr. Stanley Jaki’s philosophy of science in that it accepts Pierre Duhem’s view of science (p. 498, n. 34). Ashley describes Duhem as arguing “that natural science, as distinguished from natural ‘philosophy,’ can never do more than, like Ptolemaic astronomy, ‘save the appearances.'” (p. 220). By the phrase ‘save the appearances’, Ashley means to say that Duhem did not think that the results of physics told us anything about the world, but rather physicaly theories only helped to summarize and organize collected data of phenomena. In contrast, Ashley describes the view of the River Forest Thomist school (to which he belongs), which states that modern science does reach, in some cases, certain (and not merely probable) conclusions. It further insists that natural science is not totally distinct from the philosophy of nature and metaphysics. In an earlier piece of writing, Ashley details his problems with Catholic philosophers like Duhem, Jaki, Maritain, etc., who say that the subject of metaphysics (immaterial being) is known and thought about independently of the results of natural science, e.g., through intuition or something else. He states that these philosophers cause metaphysics to be forced to prove that its own subject matter, immaterial being, exists. Such circular justification conflicts with Aristotle’s and Aquinas’s organization of the sciences, which states that no science can prove the existence of its own subject-matter.
Because I enjoy philosophy of science and theology, I find Ashley’s criticism very interesting and I would like to evaluate it. I will begin with the obvious point that Ashley is at least right in that Duhem does repeatedly say that physical theory does not describe objective reality. In The Physics of a Believer, he says that physical theories do not touch objective reality, unlike metaphysical and religious statements: “For us the principle of the conservation of energy is by no means a certain and general affirmation involving really existent objects. It is a mathematical formula set up by a free decree of our understanding in order that this formula, combined with other formulas postulated analogously, may permit us to deduce a series of consequences furnishing us a satisfactory representation of the laws noted in our laboratories.” In other words, physical theories just help us summarize laws and data from the lab: they do not claim to describe reality. He was criticized in his own day by scholastics for saying that physics does not search after physical causes: in an 1893 article of the periodical for Catholic scientists, Revue des questions scientifiques, E. Vicaire sarcastically summarizes Duhem’s position: “Yes, Galileo, Newton, Ampère, you thought you pulled back a part of the veil that hides from us the secrets of nature, you believed you had discovered some aspects of eternal beauty. Illusion!”
On the other hand, Duhem does say other things that show he believed physics could help describe reality. As Brandon points out, Duhem thought that physics could contribute to common sense (which for Duhem is a mental faculty concerned with objective reality). Furthermore, if we take a look at Duhem’s response to Vicaire, we find an answer that could just as well be aimed at Ashley. He begins, after summarizing Vicaire’s position, by saying that there is a difference between Aristotelian vocabulary and modern vocabulary. For the modern way of speaking, which Duhem uses, physics is simply the study of the phenomena of things, noting how they behave, writing the data, discovering laws, and forming theories. On the other hand, “the study of the essence of material things as causes of physical causes” (p. 56) is a subdivision of metaphysics. On the contrary, he continues, for the Aristotelian, the study of essences is metaphysics and physics is the study of the change of material things, “that is, the modifications that the essence of material things is subject to in each passage from potency to act”. According to Duhem, what moderns call metaphysics includes what Aristotelians call physics and metaphysics. What moderns call physics, on the other hand, has no specific corresponding Aristotelian term because no such activity took place in Aristotle’s time, except maybe astronomy, which was taken by ancients much in the same way that Duhem takes modern physical theory: it simply organizes observations, it does not itself necessarily describe the reality of the motions of the heavenly bodies.
After making it clear that he uses physics according to the modern usage and not the Aristotelian, he says something on p. 60 that sounds not far from what a River Forest Thomist might say, just in slightly different words:
When, therefore, we depart from certain physical knowledge [i.e., knowledge of phenomena] (as perfect and extensive as one wants) and go from effects to causes to obtain a metaphysics, we acquire of the essence of material things a knowledge very incomplete, very imperfect; this knowledge proceeds rather by negations than by affirmations; rather by exclusion of certain hypotheses which could be made about the nature of things than by positive doctrine about the nature of things; it is only in some rare cases that, by exclusion of all hypotheses possible except one, we manage to acquire a positive document about the essence of material things.
Like the River Forest Thomist, Duhem says that we can proceed from physical knowledge to knowledge of the essences of things and in some rare cases this can even be certain knowledge. This is the same as the River Forest Thomists’ position. For although they like to emphasize that scientific knowledge can be certain, they also acknowledge that most scientific knowledge is in fact probabilistic. In The Modeling of Nature, Fr. William A. Wallace (another River Forest Thomist) says, sounding like Duhem, that some scientific concepts are mental, i.e., they merely save the phenomena but do not actually exist, such as ether, while other scientific concepts accumulate evidence for being not only mental but also objective (e.g., electrons). The sole difference is that Duhem considers certain knowledge of the essence of things to be metaphysical, whereas the River Forest Thomist would say such knowledge falls under natural science. So it is at the very least clear that Duhem does not think that all of our knowledge of the natural world is probabilistic.
This is even clearer with Jaki, as realism about the natural world is emphasized in his best-known publications. He regards metaphysics as inseparable from science, and especially a belief that there is an objective reality that science describes. Again, such views of his are not something hidden in obscure French journals: it’s a pretty important part of The Road of Science and the Ways to God, for example. In chapters 10-12 of that book, he attacks positivists like Mach and applauds Planck and Einstein for believing that science discovered universal truths about objective reality. Further, in the section of his biography of Duhem about his subject’s philosophy (chapter 9), he seems at pains to state Duhem’s realism and to qualify his talk of ‘saving the appearances.’
That finishes what I have to say about Duhem and Jaki, although I should mention that there is a lot by Duhem I have not read (as just one example, I have not read this book of his where he treats the above subject in more depth). Now, doing what I promised above – namely, evaluating Ashley’s criticism of Duhem – is actually quite tricky because exactly what Ashley is accusing Duhem of is not particularly clear. To repeat the criticism, Duhem, according to Ashley, argues “that natural science, as distinguished from natural ‘philosophy,’ can never do more than, like Ptolemaic astronomy, ‘save the appearances.'” The difficulty here is that while he admits Duhem does allow certainty, it is only in “natural philosophy” and not in natural science. What does this mean? The most likely interpretation will take into account that Ashley explicitly links Duhem’s position with Maritain’s. And Ashley says that for Maritain, natural philosophy was concerned only with the types of questions treated in the Physics and De Anima, “while all other more specific questions about nature must be relegated to a perinoetic modern science limited to a mere ‘saving of appearances.'” In other words, natural philosophy – what can be known with certainty – only studies basic natural concepts like what substance is, what change is, what the soul is, how ideas originate in the mind, etc. More specific questions, like how chemical equilibrium occurs, are not considered to fall under natural philosophy, and these are under the natural sciences, which cannot provide certain answers. Now, given that this is how Ashley describes Maritain’s position, and given that he links it explicitly to Duhem’s position, it is likeliest that this is how he also understands Duhem.
If I’m right about that, then my evaluation would have to be that Ashley puts too much weight on what Duhem says about how physical theories merely save the appearances. Ashley does not recognize that he says that we sometimes do achieve certain knowledge about nature – and not just about basic concepts, but about more specific scientific matters. In this, he in fact comes very close to the River Forest Thomists, although he says this knowledge is metaphysical whereas they would say it is under natural science. That Duhem has a different definition of physics and metaphysics is probably troublesome for the River Forest Thomists, since for them that messes up the classification of the sciences, but this does not take away from the fact that Duhem did think that physics could lead us to an objective knowledge of the physical world, albeit rarely, and in much the same way as the River Forest Thomists describe.