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Can Philosophy Prove Anything?

September 4, 2022 | by Peter van Inwagen

If science can't prove or disprove something, can philosophy?

The omnipresence of God is the reason why His non-existence can't be proved by science alone. Science can prove the non-existence only of things that would exhibit some sort of local presence if they existed—like the celestial spheres or the luminiferous ether or spontaneously generated life or the canals of Mars or people whose lives and fortunes depend on the way the points of light in the terrestrial sky were arranged at the moment of their birth.

If a medieval student had asked his astronomy teachers why we couldn't see the celestial spheres, he would have been told that they were invisible because they were perfectly transparent. But, of course, you can prove the non-existence of a locally present but invisible thing, whether it's H.G. Wells' Invisible Man or the celestial spheres. It will have to manifest its local presence in some way. Everyone agreed that the celestial spheres manifested themselves in certain regions of space by being a real solid presence there—one that would stop a comet dead in its tracks.

God isn't like that: He's not invisible by being locally present but perfectly transparent; He's invisible by not being locally present at all. The luminiferous ether is locally present everywhere, perfectly transparent (by definition), and intangible—at least in the sense that we can detect no resistance when we move a physical object through it. Nevertheless, the fact that it is locally present (even if uniformly so everywhere) has observational consequences.

The earth moves around the sun in very nearly a circle, and the constant change of direction of the earth's motion through the motionless ether would have certain consequences for how light behaves. When we look for these consequences, we don't find them, and they are of such a magnitude that, even a century ago, finding them was well within the competency of experimental physics. So, because of its local presence, the ether can have its existence disproved by science. God, however, being without local presence, is not in the business of having his existence disproved by science.

God, however, being without local presence, is not in the business of having his existence disproved by science.

If what I have said is correct, it does not mean God's existence can't be disproved. It means that the proof will have to be something other than a scientific disproof (though it may include premises that have been established by scientific investigation). It will have to be a philosophical disproof: God's not being a locally present being means that the question of the relation between any observation and any statement about God will have to be a philosophical question. If anyone ever presents any argument of the form:

We observe so-and-so and not such-and-such.

If there were a God, we should observe such-and-such, not so-and-so.

Hence, There is no God, that person has to be offering a philosophical argument because the second premise (the premise that tells us what we should observe if there were a God) will never be so self-evidently true that it doesn't need any defense. This is because a being who is not locally present is so different from the kinds of beings our mental reflexes are used to dealing with.

God's not being a locally present being means that the question of the relation between any observation and any statement about God will have to be a philosophical question

The defense will have to be a philosophical one (because science, the only other possible source of defense, deals only with locally present beings). I say this not because philosophy is above science, grander, or made of finer intellectual clay but simply because philosophy is the final home of all those questions about the general nature of things that we don't know how to deal with in any decisive or compelling way.

The Non-existence of God Argument

Very well, then: any argument for the non-existence of God must be a philosophical argument. But what does this mean? If it is a fact that any argument for the non-existence of God must be a philosophical argument, what are the implications of this fact?

I will try to answer these questions. My attempt begins with a definition. Let us say that an argument for some conclusion is a compelling argument for that conclusion if any human being who carefully considered the argument, understood its premises and the reasoning by which the conclusion was derived from the premises, and did not accept the conclusion would be positively irrational.

This definition is not so stringent as to be useless. There are compelling arguments for certain conclusions. Mathematics is full of them; one famous example would be Euclid's proof that there is no greatest prime. Here is a non-mathematical example. The great crackpot Immanuel Velikovsky (there can be greatness in crackpottery) sets out in his 1950 best-seller Worlds in Collision, a theory according to which the earth has changed its direction of spin during the span of recorded human history. (His position is not that this reversal was a miracle, as in H. G. Wells' story The Man Who Could Work Miracles. He contends that it was an event in the natural order.)

Now anyone with even the most elementary physics knowledge will know this is impossible. But let us leave physics aside. If such an event were to occur, the very least we could expect is that, at the moment of the reversal of the direction of the earth's rotation, there would be violent earthquakes on every point on the surface of the earth. (Those who are not willing to leave physics aside will realize that this statement is comparable to the statement that if a hydrogen bomb were to go off in your bedroom, the very least you could expect is that the bedroom windows would be blown out.)

So, if Velikovsky is right, there was some moment in, say, the last ten thousand years at which there were violent earthquakes all over the earth's surface. But there is a compelling argument for the conclusion that this thesis is false. (The argument is due to Isaac Asimov.) In many places in the world, there are limestone caverns decorated by nature with those remarkable structures called stalactites and stalagmites—and these structures are as delicate as they are lovely. A violent earthquake would cause them all to come crashing down. And they take hundreds of thousands of years to form.

It follows that there has been no earthquake during the last ten thousand years, at least in those places where there are stalactites and stalagmites. And so Velikovsky is refuted. And this argument is, in my sense, compelling. Anyone who understands it and does not reject at least Velikovsky's thesis that the earth changed its direction of spin in the last ten thousand years is simply being irrational. (Strictly speaking, I suppose, the scientific reasons for thinking that stalactites and stalagmites take hundreds of thousands of years to form would have to be included in the reasoning for it to be truly compelling. But I think it would be irrational to reject expert testimony on this matter, and that is what geologists tell us.)

There are, therefore, compelling arguments—against there having been ubiquitous earthquakes within the last ten thousand years, against the existence of the celestial spheres, against the existence of astrological influences. All these examples are arguments for the non-existence of various things, and I have chosen them as examples because non-existence arguments are our primary interest here. But, of course, I don't mean to imply by my choice of examples that there aren't also compelling arguments for the existence of various things. There is, for example, a persuasive argument for the existence of a causal link between cigarette smoking and lung cancer.

But are there any compelling philosophical arguments? If so, what would they be? Let us look at a philosophical argument that I will use as an example simply because I think it's a pretty good argument. First, a bit of stage-setting. Let's say that one has free will if, when faced with a decision between two or more alternatives, one is at least sometimes able to choose either of them: each is open to one. Suppose, for example, that I'm trying to decide whether to admit to the offense the police have charged me with (and of which I'm indeed guilty) or to try to brazen it out.

If I'm both able to confess and able to try to brazen it out, if both alternatives are open to me, then I have free will—at least on this particular occasion. And let's say that determinism is the thesis that the past determines a unique future. Given the past and the laws of nature, there's only one way for things to go on. It seems pretty obvious to most people that determinism implies that no one has free will. It comes as a surprise to most undergraduate students of philosophy that many great philosophers have denied this—that many great philosophers have affirmed that one can have free will even if the past determines a unique future. (Thomas Hobbes, David Hume, and John Stuart Mill are examples of great philosophers who have affirmed this.)

But here's a philosophical argument, which I think is pretty good, for the conclusion that these great philosophers were wrong—that their defenses of the compatibility of free will and determinism were, in the words of an equally great philosopher, William James, "a quagmire of evasion."

What’s True about Determinism

Consider the case in which I'm trying to decide whether to confess or to try to brazen it out. If determinism is true, one of the two alternatives I'm trying to decide between belongs to that one future that is determined to occur by what has already happened. Let's suppose that the determined alternative is confession, that the past determines that I'm going to confess. If I have free will, then, although I'm in fact going to confess, it must be the case that I'm able to try to brazen it out. That is to say, a future in which I attempt to deceive the police must, in some sense, be open to me.

But, given the past, such a future can occur only if there is a violation of the laws of nature. And how can a future in which a violation of the laws of nature occurs be open to me? How can I be able to do something such that, before I do it, a violation of the laws of nature must occur? It just seems evident that, if it was determined a million years before I was born that when a certain moment rolls round, I'll confess my crime to the police, it isn't open to me to do anything else—I'm not able to do anything else. And it just seems evident, therefore, that Hobbes, Hume, and Mill were wrong.

Well, there's a philosophical argument. As I say, to me, it seems to be a pretty good argument. But is it a compelling argument? That is if someone understands it and continues to believe that one can have free will even in a world in which determinism rules, must we conclude that that person is simply irrational? If anyone thinks this, he has to deal with an awkward fact: many very able philosophers reject this argument. I'd like to believe that the argument is compelling because, if for no other reason, I've spent a large part of my professional career defending various rather more technical versions of it.

But if I am tempted to believe this, I have to consider an awkward fact: my great contemporary, the late Professor David Lewis of Princeton University, was aware of this argument and rejected its conclusion. And I am convinced that Lewis understood the argument perfectly. And, although he once asked me not to say this, he was more intelligent than I am and a technically more able philosopher to boot. I have simply enormous respect for Lewis; I cannot adequately convey to you the depth of this respect.

After hearing one of Lewis's lectures, I once heard a philosopher say, "Lewis is so smart it's scary." And I agree. Am I to believe that Lewis was irrational—for that's what I must believe if I'm to believe that the argument I've laid out is compelling? I find I can't believe that. In fact, I find that trying to believe that is like trying to believe that the sun is green or that pigs can fly. I can only conclude that the argument I've spent a large part of my professional life defending, whatever its merits may be, is not compelling. And, I must add that Lewis is not the only philosopher I respect who rejects this argument; there have been and are many others.

Now, what of arguments for the non-existence of God—which, as I've tried to show, must be philosophical arguments? Let me lay those arguments aside for the moment. I'll make a generalization: with the possible exception of some arguments for the non-existence of God, there are no compelling arguments for any substantive conclusion in philosophy.

I offer the following argument for this generalization (and if you're waiting to catch me out in a contradiction on this point, I'll tell you right now that I don't regard this argument as compelling; I just think it's a pretty cogent argument): on both sides in every important philosophical dispute, there are highly able philosophers. Are there objectively true moral principles, or is morality an entirely subjective matter? Can a purely physical thing be conscious? Do we really know anything, or is knowledge an illusion? Has the state the right to compel us to do things that benefit others but not ourselves? Is it, in principle, possible for science to explain why there is anything at all?

For each of these questions, you can find able philosophers who will answer it Yes, others who will answer it No, and some who will say Maybe. Therefore, we must admit that a pretty strong—but not, of course, compelling—case can be made for the conclusion that there are no compelling arguments in philosophy (with the possible exception of arguments for the non-existence of God)—no proofs.

Now someone may point out that proof is a very strong word and that in the practical business of life (and in science as well), we are often satisfied with something a good deal weaker than proof. If I am apprehended walking out of a jeweler's shop with thousands of dollars worth of the shop's diamonds in a concealed pocket in my overcoat, that doesn't prove I'm a jewel thief. Maybe the concealed pocket is there for some innocent reason, and perhaps someone slipped the diamonds into the pocket to frame me.

It happens all the time in the movies. Still, as Thoreau said, "Some circumstantial evidence is very strong, as when you find a trout in the milk"—meaning that that's very good evidence that the dairy is watering the milk. The evidence I have imagined may not constitute a proof that I am a thief, but it would probably be enough to get me convicted of theft in a court of law, and it would provide anyone who knew about it with an excellent reason for thinking that I was a thief.

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