22 How Do We Know the Laws of Nature Are True?
This question might be asked in two tones of voice: “how do we know?” in the sense that maybe we don’t, and “how do we know?” in the sense of really, how do we manage to know that they are true? Let’s take that second sense first. What sorts of observations and reasoning gives us reason for thinking a supposed law of nature is actually true?
We should begin by noting that laws of nature are general statements about regularities that objects and their features must obey. Force is always required to cause a mass to change velocity. Pressure is always directly proportional to temperature and inversely proportional to volume. Overall entropy never decreases in an isolated system. Laws of nature are lawlike in the generalities they describe and in their force. They do not invoke specific particular objects, like the phone in your pocket or the top of Mt. Fuji or Larry from down the hall. Laws of nature state general truths which may apply to all relevant particulars but never focus on any specific particulars.
But, of course, all we ever see are specific particulars. And, as noticed back in our discussion of phenomenalism, we see that events happen, but we never see the necessity in their happening. So how do we arrive at knowledge of laws of nature?
One might begin by thinking that we arrive at knowledge of laws of nature by observing a phenomenon repeatedly and seeing what regularities hold. So, for example, a line of frogs comes our way, and the first one hops, and the second one hops, and the third one hops, and so on, and eventually we begin to think that maybe “all frogs hop” is true. That is a general statement, and it seems to be based on observing a string of particulars.
But there is a logical problem. It simply does not follow from the claim that one frog hops and a second frog hops and a third one hops that all frogs hop. Moreover, the observation that all the frogs observed so far hop does not demonstrate any necessity in their hopping. Consider for contrast the situation in which an old man in front of you in line at a convenience store pulls one penny out of his pocket and then pulls another one out of his pocket and then a third one… As frustrating as the experience may be for you, you should not conclude from it that all coins in the pockets of old men in convenience stores must be pennies. Your tedious experience does not allow you to boast that you have discovered a new law of nature.
Of course, doing real science is harder than just watching frogs hop or old men count out change. Scientists observe carefully, employ control groups, and run tests to find accidental correlations. But even so the logical fact remains that no number of particular observations show the truth of a general and necessary claim. Put another way, a general truth, like a law of nature, is always underdetermined by its evidence. This, by the way, is known as the problem of induction, and it was made famous (or inf