June 28, 2017

Newton and Berkeley on the Scope of Natural Philosophy

In the first two editions of the Principia, Newton makes two pronouncements about the scope of natural philosophy that appear to be in tension with one another. In the first (1687) edition Preface to the Reader, Newton writes, "the basic problem of [natural] philosophy seems to be to discover the forces of nature from the phenomena of motions and then to demonstrate the other phenomena from these forces" (Janiak 60). In the famous General Scholium added to the second (1713) edition, Newton writes, "to treat of God from the phenomena is certainly a part of natural philosophy" (Janiak 113). We know from Newton's correspondence that this was a late addition to the text (Janiak 158).

These two pronouncements are not inconsistent, for the first claims that a certain class of questions constitutes "the basic problem of [natural] philosophy," while the second claims that a different, apparently disconnected, class of questions is "a part of natural philosophy." Nevertheless, there seems to me to be a tension, for if natural philosophy is a unified enterprise then one would expect all of the questions it asks and answers to be tightly connected with its "basic problem" and unless one thinks (as perhaps Descartes does?) that force is divine action or something, then it is unclear that this further question is tightly connected in this way. Perhaps Newton thinks that a legitimate but peripheral question for natural philosophy is why God has created the specific forces that exist, rather than other forces, or perhaps he simply doesn't think of natural philosophy as a tightly unified enterprise organized around "the basic problem". This is not spelled out in the General Scholium, and indeed the paragraphs on God have the appearance of an irrelevant digression.

What I want to suggest is that Berkeley's De Motu (1721) can be seen in part as a defense of rigorous adherence to Newton's position in the first edition preface (reinterpreted in light of Berkeley's own account of force) as against Newton's remark in the General Scholium.

In the Principles (1710), Berkeley refers to natural philosophers as "Those men who frame general rules from the phenomena, and afterwards derive the phenomena from those rules" (§108). This appears to be an echo of Newton's first edition Preface. Curiously, though, Berkeley omits all mention of force, which is the central concept in Newton's physics. More generally, in the Principles, Berkeley shows very little concern about the nature or status of force, and it is unclear whether he even really recognizes its importance to Newton. This oversight is rectified in De Motu.*

In De Motu Berkeley writes:

in mechanics, notions are initially established—that is, definitions and first general statements about motion—from which more remote and less general conclusions are subsequently deduced by a mathematical method. And just as the magnitudes of particular bodies are measured by applying geometrical theorems, so likewise the motions of any parts of the system of the world, and the phenomena that depend on them, become known and determined by applying the universal theorems of mechanics. That is all that a physicist should aim to realize.

Just as geometers, for the sake of their discipline, invent many things which they themselves cannot describe nor find in the nature of things, for exactly similar reasons a student of mechanics employs certain abstract and general terms and feigns in bodies a force, an action, an attraction or solicitation, etc. which are extremely useful in theories and propositions, as also in calculations of motion, even though it would be as vain to seek them in the very truth of things, or in bodies that actually exist, as it would be to seek the things that geometers invent by mathematical abstraction (§§38-39, tr. Clarke, boldface added).

The comparison of physics to geometry in §38 is Newton's (see Janiak 59-60), but §39 takes this in a distinctively Berkeleian direction, arguing that forces are no more to be found "in the very truth of things" than length without breadth or other geometrical abstractions. The key point, though, is Berkeley's assertion that this "is all that a physicist should aim to realize."

Berkeley's position here, as in the Principles, is that "mechanical explanation" is a matter of subsumption under general laws (De Motu §37). What is new here is the recognition that these laws cannot be formulated without certain "general and abstract terms" (§7; NB: abstract terms, not ideas), like 'force'. However, although Newton defines 'impressed force' as "the action exerted on a body" by another body (Janiak 80) and Berkeley concedes that "this way of speaking is appropriate for mechanical demonstrations" (De Motu §28), nevertheless Berkeley denies that force involves genuine action, in any metaphysically robust sense. It follows from this picture that "it is the responsibility of the physicist or mechanist to provide only the rules, and not the efficient causes, of impulses or attractions and, in a word, the laws of motion; and once these are established properly, to assign the solution of a particular phenomenon" (De Motu §35). Thus, in a passage that can be read as a commentary on Newton's discussion of God in the General Scholium, Berkeley writes:

One may conclude from this that the cause of motion and rest is identical with that of the existence of bodies [i.e., God] ... To discuss God, however, and the greatest and best creator and conserver of all things, and to demonstrate how all things depend on the highest and true being, although it is the most excellent part of human knowledge, appears to belong to first philosophy or metaphysics and theology rather than to natural philosophy, which today is almost completely restricted to experiments and mechanics. Therefore natural philosophy either presupposes knowledge of God, or it borrows it from some superior science. Nevertheless it is very true that the investigation of nature provides the higher sciences in every way with excellent arguments to show and prove the wisdom, goodness, and power of God (De Motu §34).

Berkeley concludes his book as follows:
In physics, we rely on sensation and experience, which extend only to effects that are perceivable; in mechanics, the abstract notions of mathematics are accepted. In first philosophy or metaphysics, one discusses incorporeal things and the causes, truth, and existence of things ... Causes that are truly active can be known to some extent only by reflection and reasoning ... The discussion of these causes, however, is reserved for first philosophy or metaphysics. If each science is given the scope that properly belongs to it and its limits are assigned, and if the principles and objects that belong to each one are carefully distinguished, it will be possible to treat each one with greater facility and clarity (De Motu §§71-72).

In other words, Newton ought to have stuck to his original project of "discover[ing] the forces of nature from the phenomena of motions and ... demonstrat[ing] the other phenomena from these forces," since "to treat of God from the phenomena" is not part of natural philosophy (physics or mechanics). True causes, such as God, are the province of metaphysics, a distinct enterprise with distinct methods and tools and, importantly, an enterprise in which the employment of 'mathematical hypotheses' like force is illegitimate.

(Cross-posted at The Mod Squad.)

* I don't have the book in front of me, but if memory serves this point is made by Lisa Downing.
Posted by Kenny at June 28, 2017 2:23 PM
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