THREE OBJECTIONS
Objection 1: Induction and Simplicity. Even if simplicity is doing no real work in his argument for one cause rather than many, simplicity is important in his grand generalization to
gravity “in all bodies universally” in proposition 7.
For this generalization he needs his inductive Rule 3, even though he does not mention it. How else could he make the generalization? And Rule 3 Newton explicitly justifies by saying that “nature is always simple and ever consonant with itself.”Objection 2: Appeal to Alternative Hypotheses. Given the experimental and observational evidence Newton invokes, it could be the case that there are several forces operating whose resultant is what Newton calls the gravitational force. Or, it could be that even if Newton's law of gravity holds in our solar system, it doesn't operate beyond that. 'I lie reason Newton chooses the possibility he does is that it is the simplest one.
Objection 3: Newton is speculating. If what I have said in the previous section is right, and if in fact Newton does not have enough experimental evidence to justify an inference from the phenomena to his law of gravity, then Newton is speculating. He is introducing “hypotheses” without being able to “deduce them from the phenomena” If so, this goes against what Newton himself claims. Although, he admits, he cannot deduce the cause of gravity from the phenomena, he claims that he can deduce the law of gravity. It is not a speculation.[110]
My reply to objection 1 is to agree that Newton is making an inductive argument here, but to question whether any such argument is to be justified by appeal to the simplicity of nature, or to the general epistemic power of simplicity. Indeed, in the next section I offer a pragmatic interpretation of Newton's rules that avoids treating them as rules based on the assumption that nature is simple, or that simplicity provides an epistemic basis for believing in the truth or empirical adequacy of a theory or hypothesis.
Elsewhere I have argued for the claim that inductions are to be defended “locally” by appeal to the particular facts cited (e.g., by appeal to Newton's “Phenomena,” to his laws of motion, and to propositions derivable from these), rather than by appeal to general claims about the “consonance” of the universe or about the validity in general of inductive arguments.[111]My reply to objection 2 is to repeat Newton's own reply to objections of this sort in his Rule 4 and the brief discussion of it, which I quoted in section 1:
In experimental philosophy, propositions gathered from phenomena by induction should be considered either exactly or very nearly true notwithstanding any contrary hypotheses, until yet other phenomena make such propositions either more exact or liable to exceptions. 'I liis rule should be followed so that argument based on induction may not be nullified by hypotheses.
Newton is saying that if you have made an induction from the phenomena, as he claims he has with the law of gravity, then the fact that you can imagine a contrary hypothesis does not count at all against the induction. Of course, if new phenomena are discovered, the situation can change. Perhaps your inductive conclusion will need to be modified or even rejected; and, if that happens, perhaps new phenomena will be discovered on the basis of which a contrary hypothesis can be defended. But if no such phenomena are available, the existence of alternative possibilities casts no doubt on your inductive conclusion. Newton is not saying here that if two hypotheses explain the phenomena, choose the simplest one. He is saying that you should regard as true, or nearly so, a proposition that is properly induced from the phenomena, even if it is incompatible with a contrary hypothesis, so long as the contrary one has not been induced from the phenomena. I have defended this type of response in section 2.
Objection 3 is the most important of these objections. My reply is to accept it! I accept the idea that Newton is speculating at certain points in the argument.
For example, although he has empirical calculations of how fast our moon would accelerate toward the earth if it suddenly lost its inertial motion, he has no calculations of how fast the moons of Jupiter and Saturn would accelerate toward those planets, or how fast the planets would accelerate toward the sun if their inertial motions were to cease. Even without such calculations, he writes:For the revolutions of the circumjovial planets [the moons of Jupiter] about Jupiter, of the circumsaturnian planets about Saturn, and of Mercury and Venus about the sun are phenomena of the same kind as the revolution of the moon about the earth, and therefore (by [simplicity-based] Rule 2) depend on causes of the same kind, especially since it has been proved that the forces on which those revolutions depend are... [central inverse-square forces].
From his calculations of the acceleration of our moon toward the earth, Newton, invoking his simplicity-based Rule 2, infers that calculations on the accelerative rates of other moons and other planets would show, e.g., that if a moon of Jupiter were to fall toward Jupiter, at a point near Jupiter it would fall at the same rate of acceleration as any unsupported body near Jupiter would fall toward Jupiter. Just as happens with our moon, he is inferring that there is one force here, not two.
Now, a critic might say to Newton: “Sir Isaac, I need more empirical argument here. You are inferring something about the moons of Jupiter from information about our moon. Indeed, you are making a very grand generalization about all the planets that is based on calculations of our moon. Let's concede that you have shown that the forces involved in all these cases are central inverse-square forces. But you haven't yet shown that they are the same force; they could be different forces that combine to produce twice the effect of either. You show they are not different forces in the case of our moon. But unless you can produce such calculations for other bodies as well, you are just speculating.
And my reason for saying you are speculating, albeit grandly, is that at this point in your argument you invoke your dogmas about simplicity—that nature is simple and does not indulge in the luxury of superfluous causes, and that nature is simple and ever consonant with itself. You don't have sufficient experimental evidence to believe them. If you had, you would give it.”I am inclined to agree with such a critic. Newton's move here, and one later when he generalizes his full law to all bodies in the universe, should, I think, be viewed as bold speculations.[112] But that, I would claim, is not necessarily a criticism of them, viewed as speculations. Assuming they are, what Newton should have said is that he has evidence sufficient to believe that the forces responsible for other lunar, planetary, and terrestrial motions involving “falling” bodies are all central inverse-square forces. But that they are all the same force, and that this same force operates throughout the universe is not yet established. Yes, it is a simpler hypothesis than one that postulates many different forces. That it is simpler can well be a pragmatic reason for pursuing it. But that it is simpler is not evidence that the universal law of gravity is true.
If we take this stand, then what Newton should have done is what he did in his defense of his particle theory of light in the Opticks. There, he introduces his particle theory as a “query” rather than as a proposition “deduced from the phenomena.” He does so because although he can give a few empirical reasons why (he thinks) the particle theory is better than the wave theory, he cannot provide experimental evidence for it. He can only suggest it as a serious possibility. In the case of gravity, although he could provide evidence for parts of his law, he could not provide sufficient evidence to believe that there is just one force of gravity and that it operates throughout the universe. He presents some empirical reasons for believing these things, but these don't rise to the standards required for evidence sufficient to believe. Yes, it is a speculation, perhaps one that only a genius like Newton could make. But, as was the case with Maxwell's molecular speculations in kinetic theory, it is a speculation for which some empirical reasons are given, and which deserves the highest marks from various perspectives, including its unification of celestial and terrestrial phenomena, its successful predictions, its mathematical rigor, and its enormous influence.
8.