Ash trees (Fraxinus excelsior L.) are dying in the UK because of a disease, ash dieback, imported from continental Europe by trans-North Sea breezes. I like ash trees a great deal. They tend to grow straight and tall, and their symmetrical arrangements of soft green leaves are some of the most elegant foliage to be found in a northern European copse. Their wood is lovely - it's sturdy, generally straight, unknotted, and very hard. It's very evocative, with a bright white colour, visible grain, and distinctive smell (a lot of people say that freshly cut ash smells 'old'). It's also very easy to work, which was a notable characteristic for me when I used to run about the place armed with a Swiss army knife and whittling know-how. The common name in English, aesc, was once used to mean 'spear'. Its wood is particularly good for that purpose.
Not all ashes are the same. They are affected by multitudinous variables, especially wind, soil conditions, altitude, latitude, and whether boys have gone around hacking at their branches. Radiation can presumably have the same effect it does in animals and produce mutated, aberrant ash trees, and ash dieback, if withstood, can also have some effect on the later growth of the tree. Presumably, too, these could have such an impact on the development of the ash that none of the general characteristics of the tree make an appearance. There are certainly some ash trees that haven't grown straight and whose wood would be useless for making spears. There might be some whose summer growth has been too stunted, resulting in soft wood with no resilience, contrary to our expectations of ash wood. This is more likely the further north one goes.
There are also several species of ash. They aren't homogeneous, and there is genetic variation within the species that we've got. We can see this in the unfortunately low resilience of European ash trees to ash dieback, and the fact that some trees resist while others do not. We can't say that ash dieback is a threat to all the organisms covered by the name of 'European ash', although it is to most of them.
But does that mean that there's no 'ash nature', no common core of overlapping features that we can generalise about, while accepting that there might be exceptions? Does it mean that the genetics of the ash have such little impact on its development that we should refuse to generalise about the trees? Should we say that the core features of the ash can only be understood in context (which is trivially true), that every ash is different in history and even genetics (also trivially true), and therefore refuse to make any claims about the utility of ash wood in making furniture or spears or poles? Can we not communicate anything useful about ash trees to one another without noting all the variables involved in each specific example?
I don't think so.
You can generalise a lot about people. This is the basis of cognitive psychology - indeed, of all branches of psychology. There are still some questions about the specifics of what humans are like, but humans aren't exactly mysterious beings. They are intelligible to us and have been made intelligible by the application of reason to understanding them (i.e., by the scientific enterprise). Reading the essay by Steven Pinker in The New Republic and seeing the risible reactions to it, you get the impression that humans are a mysterious species whose attributes are wholly flexible - not like ash trees at all, but completely malleable by history and experience such that there is no human nature. The ash tree example is intended to show what happens if you try to apply the critics' reasoning to something other than humans. In reality, human nature is as well-understood as ash nature, and while we all understand that humans are affected by myriad variables, just like ash trees, there are in fact plenty of core features of human beings that we can talk about (even leaving aside basic stuff like physiology).
For instance, humans are typically capable of processing multiple orders of recursion. Iguanas, dogs, and ash trees can't do that; it's a product of our phylogenetic inheritance. Humans tend to have a theory of mind - i.e., they are capable of interpreting actions, whether human or otherwise, as intended by the agents, and in fact they seem to do this automatically. They do not like to experience pain and they generally don't want to die, or at least they fear death. They are capable of acting on the basis of beliefs about the world encoded in their brains resulting from sensory data, including beliefs about the beliefs of others based on observations of their behaviour. They inevitably bridge the is/ought divide, and find it hard to conceive of the idea that there is even a divide there.
They are incapable of acting against what they believe to be the right course of action, but they have divided brains that can cause them to want mutually contradictory things, so that they can give the appearance of both free will and 'weakness of the will'. Their decisions are the automatic result of the equally automatic weighing up of sometimes mutually contradictory reasons for action. They tend to want sex or procreation - sometimes both - at regular intervals. They tend to avoid sexual intercourse with close kin, i.e., those people imprinted as close kin through co-residence in childhood. They tend to care about the opinions of others, even others they've never met and will never meet. They tend to care about reputation. They care more about friends than strangers, and tend to care more about family than friends. They can learn patterns of thought and action that allow them to operate on instinct. They become bored and need new stimuli. We can see all of this in what humans do and say.
You can generalise a lot about people, and you can make plenty of predictions about what they will do in given circumstances. We'd have no realistic linguistic or archaeological models without this. We wouldn't be able to understand urbanisation or literacy without this. This is the result of the phylogenetic ancestry of human beings. To deny this - to say that there is no such thing as 'human nature' - is wrong. Yet this is exactly what the Pinker-phobes are saying.
When Tim Ingold says that, while there is no such thing as human nature, humans can't be made into whatever you want them to be, he is echoing Steven Pinker from the other side. Pinker says that humans are variable and affected by all sorts of historical circumstances and that their decisions are not entirely the result of their genetics, but also that their phylogenetic ancestry is key to understanding their behaviour. Ingold says that humans have some (presumably innate, phylogenetic) limitations that affect what they can and cannot do, but that humans nonetheless vary according to historical circumstance. The difference between Ingold's stated position and Pinker's stated position (not the anthropologists' caricature) is slim indeed.
(Ingold has also said that ethnography isn't data collection, but of course it is; his claim that it is a way of seeing 'from the inside' is a crock based on a poor and incorrect view of introspection and human capabilities. The mystical view that he takes assumes that anthropology is something other than the attempt to make human life, nature, and history intelligible. This is probably related to his fuzzy, bullshit denial of 'human nature'.)
I liked Steven Pinker's essay. I even liked the positive embrace of the label of 'scientism', because, as he defines it, it is positive. If you actually take the time to understand what he means by the word 'science' (which he uses to mean something along the lines of the application of reason to the natural world, which is believed to be intelligible) then it is genuinely hard to disagree with what he is saying. If you've decided to argue against reason, then you've already refuted your own claim by employing reason to make the argument, and once you've accepted reason you can't say that it should be suspended when examining certain phenomena. That's self-refuting and stupid.
Finally, contra that awful expletive-filled rant about Steven Pinker that is making the rounds, Pinker is of course aware of the philosophies of people like Spinoza, Kant, et al. Kant's essay on perpetual peace is a key part of his argument in The Better Angels of our Nature, and only someone who hasn't actually read his books could come to the conclusion that he is as philosophically unsophisticated (not to mention as boorish) as the author of that poorly written and moronic essay. Only someone who already hated him could think that he's as terrible as his accusers suggest he is. And, by the way, is the idea of correcting Spinoza referred to in the essay so offensive? No: it's only offensive if you treat someone like Spinoza as infallible. It seems to me that attempting to correct earlier thinkers based on what is now known is a perfect way to do philosophy, and it seems like the very best way to approach older philosophical texts - not as holy writ or solely as historical documents, but as things that could potentially be wrong and productively engaged with. It says a lot about the Pinker-haters that they disagree with this position. It may say even more that they will form of a chorus of agreement with a stupid and frankly offensive essay full of such poor arguments (can you imagine if Dawkins wrote something like that about religious belief?).
The idea that the world, including human beings, is intelligible to reason is not a scary idea and it's not a bad idea. It's just good sense. The Pinker-phobes don't even seem to know what they're arguing against.
See here for more Pinker-related shenanigans.