His original post included these thoughts, with my emphasis:
To be clear, as I understand it, all of the following is uncontroversial:
1. There is substantial evidence that IQ is heritable (which does not mean, contrary to what many blogs, as well as the HLS student, seem to think, that it has a genetic basis)...
and a bit later:
A very clear explanation of the main points is this essay by Ned Block (NYU). It is useful, in particular, in explaining why the heritability of IQ is not evidence of its having a genetic basis.
I read the very interesting essay and came away embracing the point made by Block in his conclusion - the current state of our science is such that we cannot separate direct genetic effects from indirect genetic effects and environmental factors in looking at IQ and race.
I then made the seemingly obvious point that Block's conclusion, "We don't know if X is true", is not at all the same as saying "X is false". And my rousing finish was that Prof. Leiter had made just that error, with his assertion, in (1), that IQ being heritable "does not mean, contrary to what many blogs, as well as the HLS student, seem to think, that it has a genetic basis". My position, and the position taken by Block, is that the evidence is inconclusive.
In his response Prof. Leiter focuses on the same red-headed children I excerpted from the Block essay and commits yet another error of logic (again, my emphasis):
But what the evidence for the heritability of IQ across racial groups shows is compatible with both the "IQ is genetically determined" hypothesis and the "differences in IQ is environmentally determined" hypothesis. (Indeed, as Block suggests, the evidence we have about the effect of environmental variables on IQ might even slightly favor the latter, but let's bracket that since the point stands without deciding that isue.) If a purported piece of evidence is logically compatible with P and ~P, then it can't actually be evidence for either...
Hey, he is the philosopher! However, I would note that the P and ~P notation commonly refers to a proposition and its negation. For instance, if P is "IQ is heritable by means of a direct genetic mechanism", the negation of P would be "IQ is *not* heritable by means of a direct genetic mechanism." An example of a non-negation of P would be "IQ is heritable by means of the interplay of genetic mechanisms with the environment". That would be an example of an alternative hypothesis, perhaps one of many.
As another possibly less emotive example, a dead body in an abandoned warehouse might be evidence consistent with the proposition that a pre-meditated Murder One has been committed. It might also be consistent with alternative hypotheses such as manslaughter, suicide, or accidental death. However, a dead body would not in itself be consistent with the negation of the initial proposition - I don't think many people would say that the presence of a dead body is, by itself, evidence that a murder did not occur.
With multiple competing hypotheses it is quite common to have evidence in support of more than one hypothesis. Contra Leiter, it does not follow that a particular fact is not evidence at all. However, I am having fun imagining a courtroom drama written by Leiter:
Defense attorney: If this is a murder case, where is the body?
Prosecutor: What do you want a body for? A dead body would be consistent with so many other theories that it is not evidence at all!
Defense attorney: Judge?!?
I'm done imagining; Scott Turow fans can relax now.
As to Leiter's logic, unless the only two mutually exclusive hypotheses we can develop are nature and its opposite, thereby sending "nurture" to the sidelines along with "both" and "neither", Leiter is nowhere with his negation of P and his lack of evidence.
SO EXCITED I NEGATED MYSELF! I meant to highlight Leiter's careful self-editing in his rebuttal piece:
I am sorry to have to spell this out in such tedious detail, since I'm sure it was obvious to many readers the first time around. But perhaps it will do some good, maybe even with the proverbial know-nothings of the blogosphere, who, alas, did not disappoint. This one, for example, despite actually making an effort, it appears, to read Ned Block's essay, announces with a sense of triumph that Block's essay does not support "Leiter's assertion that there is no genetic contribution." But, of course, I nowhere asserted any such thing: this right-wing blogger just made it up. "There is no evidence--literally none--that IQ differences between racial groups have a genetic basis" means what it says.
I made it up? Disingenuity alert! Nowhere in his rebutal does Leiter fully repeat the assertions I actually criticized, and have noted above. The closest he comes is this:
In response to the earlier post, various e-mailers and bloggers proceeded to cite the heritability studies (apparently not noticing my first "uncontroversial" proposition, namely, that "There is substantial evidence that IQ is heritable"),...
Let's have that one more time, because you know the motto here - No dead horse left behind, unbeaten:
1. There is substantial evidence that IQ is heritable (which does not mean, contrary to what many blogs, as well as the HLS student, seem to think, that it has a genetic basis).
Still wrong, and he said it.
WE WELCOME THIS CLARIFICATION:
We see this in an UPDATE to the follow-up post:
He [the "hapless" Yours Truly] now adduces as proof that I asserted what I did not assert my first uncontroversial proposition about IQ and heredity, to wit: "There is substantial evidence that IQ is heritable (which does not mean, contrary to what many blogs, as well as the HLS student, seem to think, that it has a genetic basis)." He thinks the parenthetical is an assertion that IQ has no genetic basis, even though it refers, explicitly, to the confusion of the HLS student (and various bloggers) about heritability and genes.
I am sure that explanation would be helpful if I understood the language in which it was written. Meanwhile, I am still toiling with an interpretation that distills to "There is substantial evidence that IQ is heritable (which does not mean... that it has a genetic basis). And we still have Block arguing that there may or may not be a direct genetic link, but the science is not settled.
TURNING UP THE HEAT: From the comments:
Leiter's position is very much like the creationists who nit pick some difficult aspect of evolution science and use it as cover for their denial. - not harvardian.
Leiter is thinking like a creationist. Or a global warming denier. Ahhhhh!
And can we muster a double-ouch? Yes, we can!
Leiter is gradually coming around to Grace's position, the one he labeled "ignorant." - Jim Ryan
He has just begun his walk down the long road to Nuance.
I NORMALLY DEPLORE CHEAP PSYCHOANALYSIS, BUT... Prof. Leiter is so convinced of the correctness of his position that he seems to have abandoned logic and even English. Geez, I just hope he doesn't get so wound up he starts driving on the left.