Here is my proposed candidate for most abused quote in the history of philosophy:
“In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, it is necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers.” (David Hume, A Treatise of Human Nature, book 3, part 1, section 1)
Hume wrote the above in 1739-40, and his admittedly ambiguous prose has ever since created a lot of needless acrimony among people who try to figure out if we can arrive at a reason-based system of morality. Hume himself seems to be skeptical of such possibility, since in the sentence just following the above quote he adds: “And I am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.”
What’s going on here? Hume’s target were theologians of the Scholastic mold, who had dominated philosophy throughout the Middle Ages, you know, people like Anselm of Canterbury, Duns Scotus, William of Ockham, and Thomas Aquinas. Hume had no patience for their approach, an attitude for which I must say I do have much sympathy. Famously, Hume attacked the Scholastics in no uncertain terms a few years later, in his 1748 book, An Enquiry Concerning Human Understanding, where he wrote:
“When we run over libraries, persuaded of these principles, what havoc must we make? If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: For it can contain nothing but sophistry and illusion.” (Section 12, part 3)
Here Hume introduced his famous fork: when we want to say something substantive about how the world works, we can do it by only two means. Either we talk about quantities and numbers (i.e., math) or we talk about empirical facts (i.e., science). Everything else is “nothing but sophistry and illusion.” Since the Scholastics didn’t concern themselves either with math or with science, you can see why Hume advises us to commit their books to the flames (presumably, only metaphorically!).
As much as I am attracted by the notion of Hume’s fork, it needs immediately to be noted that the Enquiry itself does not contain either discussions of quantities or analysis of empirical facts. And yet it is very good philosophy, because it reflects critically on its subject matter, though such reflection is certainly enhanced whenever it takes on input from mathematics and science.
That said, the bit from the Enquiry is logically connected to the one from the Treatise because their target is the same: Scholastic speculation about the nature of things unrooted in either quantitative or empirical analyses. Specifically, Hume claims that it seems inconceivable to shift from talk of facts (what is and is not) to talk of values (what we ought or ought not to do), which is what the Scholastics (and a lot of contemporary analytic moral philosophers) did (and do).
But notice that Hume is not saying that bridging the (in)famous is/ought gap is, in fact, impossible. He is only saying, reasonably enough, that if one wishes to move from one side to the other of the gap one has to show their homework: “it is necessary that it should be observed and explained.”
I’m telling you all of this because skeptic Michael Shermer is about to come out with yet another book claiming that science provides answers to moral questions, entitled Truth: What it is, How to Find it, Why it Still Matters, to be published next year by Johns Hopkins University Press. This on top of Shermer’s previous The Moral Arc, and of similar books by Sam Harris (The Moral Landscape) and Jonathan Haidt (The Righteous Mind). In a recent article in Skeptic (a Substack published by Shermer), educational psychologist John Hamby nicely summarizes the debate, which mentions yours truly several times since I can usually be reliably found on the side of critics of Shermer, Harris, and Haidt. (I’m in good company, as Hamby shows in his analysis: psychologist Marc Hauser, bioethicist Brian Earp, cosmologist Sean Carroll, and psychiatrist Sally Satel, among others.)
Although Hamby puts me squarely in what he considers the “philosophers are right and scientists are wrong” category, things are a bit more complicated than that. For one, I am by training both a scientist and a philosopher, so—unlike a number of colleagues on both sides of what C.P. Snow famously called “the two cultures”—I have neither animosity toward science nor disdain for philosophy. Second, my work cited by Hamby is pretty old (eight to fourteen years) and published in the rather limiting form of blog posts. I have since changed and hopefully refined my position on these issues, and have written a few books and a number of technical papers about them. Indeed, already in 2012 I published a book entitled Answers for Aristotle: How Science and Philosophy Can Lead Us to a More Meaningful Life. The subtitle makes it clear that I think pitting philosophy against science in these (or any other) matters is a mistake.
So let me state as clearly and succinctly as I can what I think of the is/ought gap and the science-vs-philosophy debate:
On the one hand, I believe any purely philosophical attempt to derive morals by analytical thinking—like the Scholastics did, but also Kant, or many modern analytical philosophers—is a really bad idea and bound to fail. That’s because morality, or ethics (I’m using the two terms interchangeably here, as the ancient Greco-Romans did) is fundamentally about solving the problem of cooperation, that is, of living in harmony with other human beings, because we are social animals who wouldn’t be able to flourish otherwise. Since the problem of cooperation is obviously empirical, and depends on biological and social facts about being human, armchair philosophy just ain’t gonna cut it.
On the other hand, I also believe that science, by itself, is insufficient because facts about human nature and societies underdetermine the various possible approaches to ethics. Underdetermination, which applies also to scientific theories, means that the very same facts are compatible with multiple theoretical constructs, which straightforwardly implies that empirical evidence is germane but, on its own, insufficient to allow us to choose among different ethical frameworks or to answer specific ethical questions.
