Christen's article, that I will be responding to, can be found at this link.
Christen does consider this to be the strongest non-religious argument against abortion. The problem is, he doesn't seem to understand the argument. He seems to assume it means that you were a human at all points in your life. That's part of it, but the argument states that you are *you* at all points in your life. You were human at all points, but the same *you* now is the same *you* then when you were a toddler, and when you were in the womb. Here's a more thorough exposition of the argument from identity.
Christen begins by restating his fallacious argument that there is no evidence for a soul -- that there is a difference between humanity and personhood. That's true, but irrelevant. The argument from identity is not a personhood argument. Christen seems blinded by the "personhood" discussion so that he can't imagine any discussion of abortion that doesn't break down to a discussion of personhood. Whether or not you talk about person, the argument is that you are identical to yourself through all points of your life.
Before continuing, I just want to counter Christen's false claim that there is no good evidence that minds can exist outside of a brain. This is just false. We may not have experience of minds existing outside of brains, but it doesn't follow from this that it is impossible. After all, if God exists, he exists disembodied but is able to think, create, etc. So if God exists, then it is false to say that a brain must be present for a mind to exist. There is also very strong evidence that the brain and mind are separate. The Law of Identity states that A=B. In other words, for anything true of A, that same thing must be true of B. Otherwise the two things would not be identical. But there are things that is true of my mind that is not true of my brain. My brain is physical, whereas my mind is not. Whenever I have thoughts "about" something, my brain does not change shape to become the thing I am thinking of. Additionally, as J.P. Moreland writes in his book Beyond Death: Exploring the Evidence for Immortality, "Mental events are fellings of pain, episodes of thoughts, or sensory experiences. Physical events are happenings in the brain and central nervous system that can be described exhaustively using terms from chemistry and physics." Moreland goes on, "Physical events and their properties do not have the same features as do mental events and their properties. My thoughts, feelings of pain, or sensory experiences do not have any weight; they are not located anywhere in space (my thought of lunch cannot be closer to my right ear than to my left one); they are not composed of chemicals; they do not have electrical properties. On the other hand, the brain events associated with my thoughts, etc. -- indeed, with material things in general -- do have these features."
So there is very good evidence that the brain and the mind are separate. But moving on.
Christen goes on to assert a thought experiment, that if he was struck with a virus that erased all of his memories, everything that makes him "Brandon" would be gone. But this isn't clear at all. He's confusing the memories, experiences, etc., with the experiencer of those memories, experiences, etc. What is it, exactly, that was experiencing those events? Why is he so sure that "Brandon" would be gone, instead of "Brandon" surviving without his experiences intact? In fact, with one question I can refute his thought experiment: are we then morally permitted to kill Brandon once he finds himself in that state? If not, then doesn't it seem like the experiencer is still there, even if all of his memories are gone?
Christen seems to be asserting a form of dualism here -- that Brandon is not his body, just his collection of psychological experiences. But he has not made a case for this, besides some misguided assertions that there is no brain or "soul" (he assumes there is no evidence, rather than engaging the multitude of philosophical and theological books that give evidence for a soul or that the mind is independent of the brain). In fact, Edwin C. Hui, in his book At the Beginning of Life: Dilemmas in Theological Bioethics, argues that this dualism results in the view that the physical organism can exist independently of the psychological entity, and it's the psychological entity that should be given ontological significance (in other words, the psychological entity is the one with intrinsic value, the one whose existence is important, not the physical organism). But this contradicts normal human experience. The sensations that our body experiences need the body as a subject of experiences, to experience these sensations, and the psychological component is necessary to comprehend the sensations so they can be understood as meaningful. Since the boyd and psychological components are both necessary for our experiences, then both are necessary for the "I", the person who is the subject of experiences. Since the body is a necessary component to the person, one cannot hold that the body comes to be at one time while the person comes to be at another time.
So Christen's critique here, like his other critiques, is simply misguided. He seems to want to force "personhood" arguments into these other non-personhood arguments. But this simply won't do. In fact, the argument from numerical identity argues that the fetus is identical to me, despite not having psychological continuity with who the fetus will become later. Christen fails to really engage with the argument, itself, instead just engaging with whether or not we are psychologically connected to ourselves through out our entire lives. We are not, but this is irrelevant to the argument from identity.
So Christen's statement that there are no sufficient arguments isn't surprising -- he doesn't really understand the arguments. In order to find an argument compelling, you have to understand it. But in order to adequately refute an argument, you also have to understand it. These arguments remain unscathed.