So we all know that things fall.

And most of us think that things fall because of gravity. We learned from Newton that things with “mass” are attracted to each other proportional to their masses and inversely proportional to their distance apart. From Einstein we learned that maybe it’s more complicated than that, having to do with “curved” space. But at any rate, most of us believe that things fall because of a “force” we call ‘gravity.’

But how do we know it isn’t actually fairies?

How do we know that the real explanation of why things fall isn’t that invisible fairies pull them down? Of course you might be skeptical that these fairies really exist—being invisible and, in fact, undetectable to any of our senses. But the same goes for gravity!

No one has ever seen gravity, or heard it, or tasted it, or touched it, or smelled it, or (I would argue) even felt it. (We feel the difficulty of holding something heavy up. But we don’t feel what is making that difficult—the gravity itself—if there is such a thing.) But most of us still believe in gravity.

Why?

How can we justify that belief?

How does the evidence we have—the things we can observe with our senses—give us good reason to believe in this thing that we can’t observe with our senses?

But the same goes for nearly everything science tells us about. Science proposes theories to explain things. And those theories inevitably posit the existence of things which are not “observable” to our senses.

No one has ever seen an electron, a magnetic field, birds evolve from reptiles, the big bang, the motion of tectonic plates, what is going in inside the sun, how granite is formed, etc., etc. (Those who mock religious beliefs on the basis of the observation that they believe in things that we cannot see or otherwise detect with our senses should be aware that by this standard they must also reject pretty much everything science tells us as well.)

Of course the reason we believe in gravity, electrons, magnetic fields, the distant past, dinosaurs, etc., is that the theories according to which these things exist “fits” what we do observe with our senses. What we observe is what we would expect if those theories were true and those things actually exist(ed). And so these theories and their claims that these things exist(ed) explains what we observe.

The gravity theory explains why things fall (and a lot more), the electron theory explains electrical phenomena, magnetic theory explains why those iron filing form that pattern in the presence of a magnet, and the supposition that there were dinosaurs explains all those fossils we find. What we observe is what we would expect to see if the theory were true and those things actually existed. So that seems to give us good reason to believe in the real existence of all those things—even if we can’t see, or taste, or touch, or hear, or smell any of them.

But here’s the enormous fly in the ointment:

For any theory that “fits” what we observe, there are in principle always infinitely many other theories which also fit that evidence just as well. If I design my fairy theory to match the gravity theory—fairies pull things toward each other, proportional to their masses and inversely proportional to their distance apart—the fairy theory will fit all my evidence just as well as the gravity theory.

So how do I know this particular theory—the gravity theory—is the right one, and not another—the fairy theory? How do I know it’s really gravity and not fairies?

This is the fundamental puzzle of Philosophy of Science:

How can we really justify belief in these scientific theories, when we cannot directly observe that they are true or that any of the things they posit actually exist?

I’ve got my own view of how to answer the puzzle, but for now, I’ll just leave you with the puzzle.

Enjoy.

-September 2017