IQ, abortion, and euthanasia

Peter Singer's argument for infanticide and euthanizing the senile and developmentally disabled is that IQ ranges along a continuum according to maturity and species. According to him, some higher animals are smarter than newborn babies, the senile, or developmentally disabled. Of course, that's a horrendous justification for infanticide and euthanasia, even if it were true. But in addition, here's a challenge to his operating assumption:

And what is rationality? Let us begin to try to answer this question by considering another question asked by the philosopher Ludwig Wittgenstein: "We say that a dog is afraid his master will beat him; but not, he is affraid his master will beat him tomorrow. Why not?" The beginning of the answer to this question is that the idea expressed by the word "tomorrow" is wholly foreign to the mental world of the dog.

This idea…is well expressed by a bit of verse by W. V. Quine:

The unrefined and sluggish mind

Of Homo javanensis

Could only treat of things concrete

And present to the senses.

A rational being is a being that can do the following:

Represent to itself complex states of affairs, including non-actual states of affairs, that are strikingly remote from its present sense-perceptions. For instance…finding new ways to dispose of the refuse that feeds the rats that carry the flees that are infected with the bacterium that cause the plague. It can believe that certain states of affairs are actual and that others are non-actual. It can desire that certain states of affairs be actual and others non-actual…("I am trying to imagine what our life will be like if we really go ahead and have a child")…It can sort states of affairs into the categories of "probable" and "improbable"…It can devise plans of action that draw on its beliefs about which states of affairs are actual and non-actual and probable and improbable…It is capable of recognizing other beings as having all these capacities, and it is capable of communicating to those that do facts and orders and questions related to states of affairs it represents to itself…

Rationality marks a great divide, a discontinuity between humanity and the beasts. It is wrong to suppose that there is something apes and elephants and beavers have a little of, and we have more of, and that as a consequence, we are rational and they are not. 

It is not that we are "more intelligent" than, say, apes, and that that is why we are rational and apes are not–as Alice is able to solve word-analogy problems and spatial-relation problems faster than Alfred because she is more intelligent…We may indeed be more intelligent than apes; indeed I suppose we are. But if so, that is not why we are rational and apes are not. If there is a connection, it goes the other way: we are more intelligent than apes because we are rational and therefore have more use for intelligence-for intelligence, if it is anything, is the ability to manipulate mental representations of states of affairs in various useful ways, and we have a lot more, and a lot more complex, representations to manipulate than apes do….Human beings who are of subnormal intelligence owing to injuries or generic defects do not have minds at all like the minds of apes, any more than apes of subnormal intelligence have minds like the minds of elephants or beavers. Rather, they have human minds that are of diminished capacity in respect of dealing with the demands of life in a human community. P. van Inwagen, Metaphysics (Westview Press, 4th ed., 2015), 184-86.