This is my third of five posts in a series on the Argument from Consciousness. Again: The Argument from Consciousness begins by presenting properties of consciousness which cannot in principle be explained on a naturalistic ontology. It then argues that it is credibly probable that agents with these mental properties will exist if there is a God but incredibly improbable that they would exist if there is not. In other words, the existence of conscious agents with mental properties that cannot in principle be reduced to the physical implicates the existence of a Nonphysical Conscious Agent as their originating cause. My first post in this series discussed qualia and the second intentionality. In this post I will be discussing a property of mental states which philosophers of mind call, “privileged access.”
The most essential property of mental states is also the most problematic for naturalism: their personal immediacy to the subject who experiences them. “A mental property,” as Oxford Professor of Philosophy Richard Swinburne puts it, “is one to whose instantiation the substance in which it is instantiated necessarily has privileged access.” To help us understand why this is a problem for naturalism, Swinburne invites us to consider the following thought experiment. It is a helpful though not essential preliminary to what follows to note that people can enjoy a relatively normal mental life with only half a brain—after a procedure known as a “hemispherectomy.”
Suppose, firstly, that Swinburne is involved in a car accident that destroys his body but leaves his brain intact; suppose, secondly, that this occurs at a future date when brain transplants are feasible; suppose, finally, that a whimsical surgeon is responsible for the treatment of Swinburne and decides to perform a bizarre experiment: He will transplant the left hemisphere of Swinburne’s brain in one donor body and the right hemisphere of his brain into another donor body. Let us refer to these two new bodies, each of which contains one half of Swinburne’s brain, as Person A and Person B. The operation is a success. Person A and Person B recover and both somewhat resemble Swinburne in terms of character and memory.
The question arises whether Swinburne has survived the operation. The claim that Swinburne is now both Person A and Person B is eliminable by a law of logic known as the identity of indiscernibles.  Very simply expressed: If Swinburne is mentally identical to Person A and Person B, then Person A and Person B are mentally identical to each other and are therefore the same person—which they are not. The remaining possibilities are that Swinburne is Person A or that he is Person B or that he is neither because the operation destroyed him.
Reflection on this thought experiment shows that, however much we know about what has happened to Swinburne’s brain (“and we may know,” Swinburne emphasises, “exactly what has happened to every atom in it”) we do not know what has happened to him. And this is important because whether or not Swinburne survived the bizarre experiment is an objective fact about the world—a fact that it will not be possible to know by either the most thorough cross examination of Person A and Person B or the most exhaustive naturalistic description of their respective hemispheres. And so an exhaustive naturalistic description of the universe leaves something essential out of account; namely, who experienced which brain states.
What arguments of this sort bring out is the “privileged access” of the subject to his own mental life—what Searle calls the, “first person ontology.” “Others,” Swinburne writes, “can learn about my pains and thoughts by studying my behaviour and perhaps also by studying my brain. Yet I, too, could study my behaviour (I could watch a film of myself; I could study my brain via a system of mirrors and microscopes) just as well as anyone else could. But I have a way of knowing about pains and thoughts other than those available to the best student of my behaviour or brain: I experience them.” And what makes a mental event a mental event is not the public knowledge captured by naturalism but precisely this private knowledge that naturalism cannot possibly capture.
This is the third property of consciousness that is insusceptible of reduction to the physical
 The Identity of Indiscernibles, also knows as “Leibniz’s Law” after its formulator Wilhelm Gottfried Leibniz, is a principle of analytic ontology which states that no two separate entities can have all their properties in common. The fact that Person A and Person B are physically distinct should not mislead us. Swinburne is concerned not with the body and brain per se but with the continuity of the personal identity and mental life of preoperative Swinburne—whether this is transplanted into either or neither of the postoperative bodies. It is obvious that the continuity of identity essential to personhood could not survive division or (due to Leibniz’s law) be doubly instantiated.