Chapter One – The Reach of Explanations

What is science? One can hem and haw over a definition that tries to capture all the varied facets of science. But because of Deutsch’s book I have an answer that is at once elegant and simple, and yet also makes a strong statement about the world.

Science is the search for good explanations.

So what are good explanations, and how do we search for them? A good explanation is one that fits all the known facts, and is also (and this is the crux of Deutsch’s book-long argument) hard to vary. A bad explanation, by contrast, might account for the facts, but it is easy to vary, and this is its downfall. Myths and other supernatural tales are easy to vary, because the details of the myth are only superficially linked to the explanation. This makes them, always, bad explanations. Not because they get the facts wrong. One can always alter a bad explanation to fit new facts because a bad explanation is easy to vary. And this is what makes it bad.

By contrast, a good explanation has nowhere to go if facts contradict it. Because the explanation is hard to vary, if a fact comes along that doesn’t fit the explanation, then the explanation is wrong – or at best, incomplete. In this way, good explanations can become bad, because they no longer fit the facts. But bad explanations can never go the other way until they themselves change to become hard to vary.

Where do these explanations come from? They arise from the human mind. We take experience, imagination, snippets of other explanations that have worked elsewhere, stir them around, and come up with an idea, a conjecture. We then criticize the conjecture, molding and shaping it until it fits all the known evidence. If we are creating good explanations, we also mold and shape the explanation in another way, linking the explanation to the evidence in such a way that the explanation becomes hard to vary.

And then something amazing happens. For some, though not all, good explanations, a new property arises. That property is reach. Some good explanations will reach beyond the narrow set of phenomena they were designed to explain. They will explain phenomena that may have seemed unrelated. They might even make predictions about phenomena that have not been observed. Some very few explanations (Newton’s and Einstein’s explanations of gravity are two examples) will have “universal reach.” These explanation will stretch not just far beyond their initial application, but infinitely far.

It is this unexpected reach of good explanations that makes progress possible. And it is this idea that Deutsch will explore through the rest of the book.