# What Makes a Good Explanation?

Today, we all know that the Moon’s gravity causes the tides, but what did people think before Sir Isaac Newton discovered that gravity is a universal force? I recently heard Jonathan White interviewed on NPR; he is the author of Tides: The Science and Spirit of the Ocean. It turns out that serious people used to hold all manner of fanciful explanations for the tides. Three that I remember from the interview are:

1. A woman is lifting her skirt and lowering it.
2. A very large beast in the depths of the ocean is breathing in and out.
3. The rays of the Moon heat rocks below the ocean, which causes the depths of the ocean to boil. Boiling in the region below the Moon causes the water level to rise.

Of course, these explanations are all wrong but some of them are better attempts at the truth than others. Put yourself in the time when the correct answer was not known. Which explanation would you prefer, and why?

The first explanation, that a woman causes high tide by lifting her skirt, is winsome Alaskan mythology but falls flat as a scientific explanation because someone ought to have seen the woman by now, but nobody has. Not to mention that nobody has seen any woman whose skirt is large enough to move that much water.

The beast under the ocean is a little more plausible because we must admit that the ocean contains many things we have not seen. A giant beast there seems more likely than a giant woman standing where we should have seen her. No less a luminary than Leonardo da Vinci thought the ocean beast was plausible; according to Jonathan White, da Vinci even attempted to calculate the size of the beast’s lungs.

Now it seems that we’re getting somewhere, doesn’t it? We are trying to calculate something, and if the calculation shows that the beast would have been so large as to be impossible, perhaps because there could not be enough food in the ocean to support it, we can reject the hypothesis and try something else.

The last wrong explanation, that the Moon’s rays heat rocks, is to me the best. It neatly explains the data we have: that the tides correlate with the position of the Moon. It builds on the knowledge we already possess: that light-giving bodies such as the Sun are capable of heating things at a great distance; that some things are more susceptible to heating than others; and that boiling water takes up more space than water at rest. Best of all, it introduces no implausible beings.

What I have said is nothing new. It’s just a particular case of Occam’s Razor: we should prefer the explanation with the fewest and most plausible assumptions beyond what we already know.

Now let’s consider a larger question: how did the universe come to be? In modern America, two explanations are the main contenders:

1. A God created the universe by speaking each aspect of it into being.
2. Our universe was born of a quantum fluctuation. It is like a bubble in an eternal froth of universes that are popping into being all the time. This is but one of several hypotheses that posit a multiverse.

As with the explanations for the tides, it is possible that both of these explanations are wrong. However, you may still have reason to prefer one over the other.

At a meeting of philosophically minded friends last week, one person said that because neither explanation is testable, they are on equal footing.

What do you think? Are they on equal footing, or do you prefer one of them? Why?

Next time, I’ll give my opinion.