Why don’t you include the skeptic’s email?

Just for the historical record, here is what the philosopher in question sent to Diana in response to the message above. The philosopher has no record of receiving a response. Surely a response at this point would be enlightening to all.

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First let me reiterate that I’m not skeptical that there is evidence for this diet. My main question is whether this evolutionary argument counts as evidence, or at least as significant evidence. If it doesn’t, then I think a lot more emphasis needs to be placed on the other kinds of evidence. I realize you’re not saying that the diet is a certainty. At best I think knowledge of a diet can count as a strong possibility. Just so it’s clear: a strong possibility is a working hypothesis that can and ought to guide one’s thinking and action in significant ways, so this isn’t dismissive of the hypothesis.

There is much to say about your response. I could say more about why I think no dietary knowledge at this point could count as better than a strong possibility, but let me just focus on the status of the evolutionary argument, and the very excellent question you asked me about it. The question was fruitful because it helped me to clarify my views on this subject, and when I tried to answer the question, many of my suspicions were confirmed.

> As for the evolutionary argument, let me ask you the following… > Imagine that you were given a dog to care for, but you’d never seenor > heard of a dog before. Would you say that the fact that dogs are very > close relatives of wolves is irrelevant to the question of what you > ought to feed this dog? Wouldn’t that evolutionary fact suggest that > the dog needs meat, meat, and more meat — not tofu or corn or alfalfa?

I would say *yes*: if I didn’t know anything about dogs, and if I knew that dogs were descendants of wolves, then this evolutionary fact could provide evidence for what the dog should eat.

But there’s a really important additional assumption here: knowledge of the evolutionary fact would provide this evidence, IF I already knew *what it is good for the wolf to eat.* If I knew that, knowledge of evolution would make knowledge of the wolf’s diet relevant to what the dog should eat.

So, we do know that we are descended from Paleolithic human beings (I’ll call them “cavemen”). If we knew what was good for these people to eat, then this would be relevant knowledge to establishing what we should eat. It wouldn’t be definitively relevant (since some changes might have occurred to us since our descent from cavemen), but it would be highly relevant.

So the important question is: do we know what was good for cavemen to eat, whether what they ate was best for their flourishing? Notice that knowledge about what is good for some organism to eat is knowledge of a causal claim. To say X is good for Y to eat is to say that eating X causes Y’s flourishing.

We probably know a lot about what cavemen *did* eat (there’s only so much that would have been available to them). And knowing what they did eat is also relevant to knowing what was good for them to eat. Obviously what they ate was *good enough* for them to survive and reproduce and leave us as descendants.

I do know about the evidence suggesting that cavemen were bigger and stronger than many modern human beings, which is *some* evidence that what they ate what was good for them. But notice that the evidence for what was good for them is evidence that is independent of the theory of evolution itself. That theory doesn’t tell us what our ancestors ate or that it was completely good for them, it only tells us (at best) that they are our ancestors, and that they ate something good *enough* to leave us as ancestors.

Notice that the theory of evolution does not tell us that any adaptation is optimal. Evolution itself is not a goal-directed process. It is not true that any trait a being happens to inherit is optimal for its flourishing in an environment–it does not even mean the trait is “progress” compared to its ancestors. The traits we inherit from ancestors may be no good for our environment, and we may die off because of it. That’s as true for us as it was true for cavemen. It’s especially true for cavemen if they were, as Christian emphasizes in his post, rational men, and therefore in possession of free will. What they ate would have been a product of their choice as well, and could therefore have been a good or bad choice, depending upon facts about the nature of their life and environment. You could push the question back to pre-rational animals, but then if their diet was just a matter of preprogrammed behavior, this still does not mean it was optimal for them in their environment.

So what kind of evidence would it take to know that the cavemen were eating an optimal diet, or one that was improved over their ancestors?

I don’t fully know: it’s a complicated biological and medical question. It is complicated in large part because it’s a question of cause and effect. We know cavemen had a diet and that it would have had some effect on their flourishing, and it would have had a significant effect. But how does this factor compare to other factors bearing on their flourishing? Why was it their diet and not exercise, lack of predators, lack of competition with other tribes, etc., that resulted in whatever degree of flourishing they had? Perhaps it really was their diet, but it is an exceedingly difficult task of inference biological and historical inference to establish that.

Here, I think, is the really crucial point: to gather evidence for whether cavemen were eating optimally, we have to first know whether cavemen were flourishing, and then identify its cause. Cavemen are very far away from us, historically. It is hard enough to know whether they were flourishing. I know there is some evidence for this, though it is based on a small number of surviving skeletons, and there is a serious question about how representative of the species these skeletons could possibly be, especially given that the stronger ones are more likely to have been buried well, etc. But what about determining the cause of their flourishing? This is harder still, because we can’t experiment with cave men to assess the relative weight of the diet factor vs. others. Everything we know about them is a product of inference.

There is one way we could know more certainly what the effect of diet on caveman flourishing would have been: if we already have a general theory of diet that tells us precisely what force the factor of diet has under any given environmental circumstances. If we knew where to look to determine the circumstances of early man, this would tell us how to find evidence of the effects of his diet under those circumstances. But where are we going to get that general theory of diet? We would need to have the theory already to establish that cavemen were flourishing because of their diet. But then we could not use knowledge about cavemen as evidence for a general theory of diet.

But of course it is precisely a theory of diet we are trying to establish.

(The point is not that we need theories to establish knowledge of cause and effect. That is the opposite of a proper theory of induction, according to which we get inductive theories by first observing simple causal connections. But obviously we can’t observe anything about cavemen directly, since our entire knowledge of them is a product of inference.)

We could get evidence for a general theory of diet by observing contemporary diets, though even here it is not direct observation because the body is a complicated mechanism with many variables affecting its flourishing. Still it is far easier for us to assess the causal role of our diet on *our* flourishing, since we are here now and can do experiments on ourselves (or on animals). But if we could get the theory that way, we would obviously have no need of historical knowledge to get it. On the contrary, we need contemporary knowledge to establish the theory, to establish any historical knowledge about the success or failure of past diets.

So I think there is an important kind of hierarchy violation involved in the attempt to determine the best diet for us based on knowledge of the best diet for cavemen. We couldn’t know whether cavemen were eating the best diet unless we could establish a causal connection between their diet and their flourishing. Perhaps we can do that for them, but if we could, and if we knew where to look for evidence of the effect of their diet on their flourishing, it would probably be because we already had a theory of diet informed by our better knowledge of the effects of our eating on our flourishing. But we don’t have that theory–it’s exactly what we’re looking to establish.

I don’t think that this means that thinking about diet and medicine in general in evolutionary terms is totally wrong. It is perhaps a useful heuristic device, an interesting thought experiment we can perform if we already have a lot of good evidence for a particular diet, which might summarize the results of that evidence. So the big question is whether we have that other evidence. I don’t think the evolutionary argument is itself is a form of evidence for the reasons given above, and so it and doesn’t lend independent support to a hypothesis about diet. That’s why I think contemporary studies are so important. To whatever extent we have them, they are our main form of evidence for the effectiveness of any diet. If they don’t tell us very much, then this limits the degree of certainty with which we could claim any knowledge of diet.

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There was, incidentally, also a long thread of discussion on this topic where the philosopher in question’s views were defended by NS:

https://philosophyinaction.com/blog/?p=5112

See especially this excerpt, the bulk of which went unanswered in the discussion thread:

“But Aristotle knew better. He knew that any number of sophistical puzzles could be dissolved (and resulting rationalist solutions obviated) by distinguishing between the order of knowledge and the order of explanation. In his terms, this is the difference between “prior to us” and “prior by nature.” On his view, there is a distinction between knowing *that* something is the case, and knowing *why* something is the case. One can and often must know that something is true before one knows why it is true.

To use Aristotle’s famous example. One can and must know that thunder is a noise in the clouds before one can know why thunder is noise in the clouds (because of the extinguishing of fire, he said). Or to use another of his examples: one can and must know that there is an eclipse before one can know why there is an eclipse (because the earth occludes the rays of the sun).

How does one know that something is true before knowing what causes it to be true? In Aristotle’s examples, one just observes: one hears the thunder and one sees the eclipse. But there are ways of knowing beyond direct observation that involve inductive generalization and inference to the best explanation which do not yet involve explanation in terms of a master science. Evolution is the ultimate master science of biology, but there is plenty we can know in biology and in physiology before we know what explains it. I see no reason for thinking that one can’t know what is good to eat without knowing *why* it is good to eat in terms of a master science.

And I see plenty of reasons for thinking why we *cannot* know why various things are good to eat in terms of biology without first acquiring a theory of nutrition based on physiological experiments performed in our own time. This is what happens in every other science. In order to know why the continents moved apart as they did, we first had to acquire theories of geology and electromagnetics and vulcanology, to understand how rocks and electromagnetic fields and volcanoes actually work, in order to know how they actually would have worked long ago to explain their present arrangement.

Evolution is a theory for explaining *why* organisms function the way they do. It is not a theory for proving that they so function. We do not need to–and cannot hope to–prove that certain things are good for us by proving first why they are good for us. We don’t and can’t any more than we can hope to prove that the moon is in eclipse by first proving why it is.

Or to use an example even closer to your position (because yours involves treating as a premise knowledge of distant eons: we cannot prove that the Earth is round by deducing it from knowledge of what caused the Earth to be round millions and millions of years ago. To give such an explanation in terms of the gravity of dust clouds, we would first need the theory of universal gravitation–which couldn’t have been formulated without first knowing that the Earth was round.”

https://philosophyinaction.com/blog/?p=5112#comment-302700346