Why are scientific laws unverifiable? Hume’s answer was that no finite number of observations, however large, can be used to derive an unrestricted general conclusion that is logically defensible. If I say all swans are white and posit that as a scientific hypothesis, how would I go about verifying it? By checking out swans. A million swans. Or ten million. Based on this I can say confidently that all swans are white. Hume’s point is that I don’t really know this. Tomorrow I might see a black swan, and there goes my scientific law

This is not a frivolous example. For thousands of years before Australia was discovered, the only swans people in the West had seen had been white. Consequently, the entire Western world took it for granted that all swans were white, and expressions like “white as a swan” abound in Western literature. It was only when Europeans landed in Australia that they saw, for the first time, a black swan. What was previously considered a scientifically inviolable truth had to be retired.

At this point one might expect today’s champions of science to start patting themselves on the back and saying, “Yes, and this is the wonderful thing about science. It is always open to correction and revision. It learns from its mistakes.” Sure enough, Carl Sagan praises scientists like himself for their “tradition of mutually checking out each other’s contentions.” Sagan’s view is echoed by Daniel Dennett, who writes, “The methods of science aren’t foolproof, but they are indefinitely perfectible…. There is a tradition of criticism that enforces improvement whenever and wherever flaws are discovered.”8

To say this is to miss Hume’s point, which is that science was not justified in positing these rules in the first place. All scientific laws are empirically unverifiable. How do we know that light travels at the speed of 186,000 miles per second? We measure it. But just because we measure it at that speed one time, or ten times, or a billion times, doesn’t mean that light always and everywhere travels at that speed. We are simply assumingthis, but we don’t know it to be so. Tomorrow we might find a situation in which light travels at a different speed, and then we will be reminded of black swans.