Unlike the humanities, including philosophy—where the idea of progress is a controversial topic—it is an essentially indisputable fact that science makes considerable progress over time. Why this is the case—and how science actually works—is what Michael Strevens seeks to explain in The Knowledge Machine

The basic argument is that scientific knowledge grows through the application of the “iron rule of explanation,” as Strevens calls it, that demands that all scientific argument be settled by empirical testing alone, and that the results of empirical testing are to be recorded in formal scientific journals for future reference and use. 

The iron rule is peculiar in the sense that it demands adherence to empirical testing and does not consider the relevance or significance of any non-empirical knowledge, whether philosophical, religious, spiritual, or aesthetic. While individual scientists are free to theorize in whatever manner they like—and are swayed by the same philosophical, moral, and political influences and biases as everyone else—the iron rule of explanation guarantees that formal arguments are presented without reference to any of these ancillary considerations. 

The net effect of this “procedural consensus,” over time, is what Strevens refers to as “Baconian convergence,” or the idea that repeated empirical testing over time converges on the one theory that best explains all the accumulated data. This is why physicists, over time, have eventually come to accept the legitimacy of the general theory of relativity, for example, whereas a religion like Christianty schisms permanently into a thousand different parts. 

The reason for this is that philosophical and theological reasoning, while attempting to be more ambitious and all-encompassing, has no ultimate method of verification or falsification through testing. If I think, as Isaac Newton did, that Jesus was created by God and subordinate to God, whereas you think that Jesus and God are one and the same, how are we supposed to resolve this philosophical difference? I can provide my logical and coherent reasons and you can provide yours—along with our respective interpretations of the relevant scripture—but without a procedural consensus whereby we can ascertain the truth beyond mere logic, there is no way to settle the argument.

On the other hand, if I believe the Newtonian theory of gravity is correct and you believe the Einsteinian theory is correct, we can (if we were capable) settle the dispute by measuring the angle that light is bent by the sun’s gravity during a total solar eclipse, as Arthur Eddington and others did. Newton’s theory predicts one measure; Einstein’s predicts the other. We can both agree to what the measurements will tell us beforehand; then, after the experiment is conducted and the measurements are verified, we can settle the dispute. 

Of course, as Strevens points out, it’s not exactly this simple. An individual scientist must still engage in “plausibility rankings” and determine how to weigh conflicting evidence. There is a strong element of subjectivity in the interpretation of evidence and the process is far from completely objective. But the main reason why science is effective is not due to the unwavering rationality of any individual scientist; rather, it is attributable to the process of several scientists over time abiding to the iron rule of explanation—and publishing detailed empirical findings—that allows the process of convergence to occur over time and the correct theory to materialize. This is why science advances, and why we are now able to launch satellites into space and communicate with each other around the globe electronically at the speed of light. 

So far, so good, but why did the iron rule of explanation—which has proven to be so effective—take humanity so long to develop? Why did it develop in 17th-century Europe and not, for example, in ancient Greece or China? The reason is, according to Strevens, that the iron rule is, at bottom, irrational. It asks the practicing scientist to effectively ignore all other forms of human inquiry that is not strictly empirical. This would have seemed absurd, especially in ancient Greece, the birthplace of philosophy, or in Medieval Europe, obsessed with theology as it was. To give up all philosophical and theological reasoning in the attempt to explain how the world works was too radical an idea for most times and places.

This is why science had to wait for the peculiar historical and cultural circumstances of early modern Europe. Only then, and only over time, did it begin to make paradoxical sense that knowledge of the world can only grow by significantly limiting its scope to empirical testing and data alone. Since then, the iron rule of explanation and its procedural consensus has resulted in Baconian convergence and a growing and sophisticated understanding of the workings of the world.

Strevens, I think, has hit on something profound in this book, and his explanation for how science works is ultimately convincing. However, I take some issue with the title of the book and on his calling the process of scientific discovery irrational. 

The iron rule is not, in itself, irrational; its tremendous success over the last few hundred years should attest to that. By limiting scientific argument to empirical data alone, our knowledge of the world has increased astronomically in a short period of time. 

The irrationality, then, does not lie in the iron rule itself; rather, it lies in the belief that the iron rule applies to problems outside the realm of science. If you believe that philosophical, ethical, and political problems can be solved with empirical argument alone, then yes, your overextension of the iron rule is indeed irrational. 

But if you limit the scope of the iron rule to scientific, empirical problems, then there is nothing irrational about the rule because there is nothing in the rule that says you cannot compartmentalize scientific problems. Isaac Newton should have demonstrated this; he abided by the iron rule in his scientific work while simultaneously pursuing other philosophical and mystical pursuits. There is nothing irrational about this. The irrationality, rather, comes from someone like Stephen Hawking, who said that “philosophy is dead” because he couldn’t apply the iron rule to philosophical problems—problems it is not meant to address. 

There is no “theory of everything”; reality is complex, like a six-sided cube you cannot view all from the same perspective. Different problem types require different approaches, and science has developed, according to the iron rule, its own successful approach. Philosophical, historical, ethical, legal, and political problems all have their approaches as well, and, while they all influence each other, no single domain has authority over all the others. 

Science has simply limited its scope to empirical testing to solve certain kinds of problems. While the scientist that thinks this particular approach can solve all types of problems is certainly irrational, scientific problem solving, led by the iron rule, is not. 

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