It is worth noting at this stage that George and Tollefsen do not support a dualist conception of persons. In fact, the refutation of dualism is a key move in their defence of the conception threshold of personhood. We will see more on this presently. Still, we can see why those who subscribe to some form of dualism could be committed to the punctualist thesis about the emergence of personhood. ‘Immaterial substances’, like souls or Cartesian egos, are not usually thought to obtain in degrees or admit of vagueness at the margins. They are either there, or they are not. The immaterial substance view is, needless to say, difficult to prove or disprove. Many people have great difficulty believing that once they come to exist as a subject of consciousness, they, or their essence, could ever cease to exist, even once their body and brain are dead and gone. But the mere fact that our imaginations falter when contemplating our own disappearance from existence is not good evidence for the belief that persons are immaterial substances.  Even so, it is difficult to imagine what a hard disproof of that proposition would look like. How could one show that intangible person essences do not exist?
Convincing arguments have nonetheless been proposed that challenge the dualist conception of persons. One possible problem, pointed out both by philosophers and scientists, is that the ‘immaterial substance’ belief is not required to explain the manifestation of typical ‘person-like’ traits, such as self-awareness and rationality, in human beings. Neuroscience has an explanation for the manifestation of these capacities, based in the physical properties of the human brain. This is why those capacities are destroyed or damaged when, for instance, brains are injured in accidents, or fail to develop properly because of congenital defects. One puzzle for the dualist view asks why it is, if the mental states possessed by persons (like selfawareness and intelligent thinking) are produced by an immaterial substance transcending human anatomy, that those states are affected by purely physical changes, like brain injury or drug use. Presumably, the immaterial substance thought to be responsible for mental states is not affected itself by physical changes.5 We might wonder, therefore, if dualism is true, why physical events that happen to the brain would alter those capacities.
A possible dualist answer to this problem might be that, on the dualist conception, the psychological capacities of the animal being in which person substances reside are in fact separate from the immaterial person substance. They are possessed by the animal, the physical being, not by the person. Thus, we should expect them to be affected by physical events. This does not imply that dualism is false.
But this answer introduces a different problem for dualism highlighted by philosophers of mind. This concerns what dualism must hold about the conscious life of the human animal that the ghostly substance inhabits. Is the suggestion that the ‘host’ human body, complete with a sophisticated brain, is not itself a thinking being? If this is true, and if only humans are persons, then humans are the only sophisticated mammals whose large cerebral cortexes do not generate conscious life. On the other hand, if both the immaterial ‘person’ substance and the ‘host’ human animal are thinking beings, it follows that every mature human being possesses not one but two subjects of consciousness, a notion which is quite extraordinary. How would each of us even know if we were the thinking person or the thinking animal? There would be no way of telling. This is known as the ‘too many thinkers’ problem.
These may be good reasons for doubting the dualist account of personhood in the absence of compelling evidence for believing it. However, it is not my intention here to make a firm case against dualism. This is because it may not even be necessary to establish the falsity of the ‘immaterial substance’ belief in order to refute the punctual- ist thesis, or challenge its use in defence of the conception threshold of personhood.
First, even if the dualist picture of persons is correct, it is still not entirely clear that it entails the punctualist thesis about the beginning of personhood. Let us assume for the sake of argument that persons are immaterial conscious substances that inhabit and animate human bodies. Must such substances come into existence immediately and wholly? Could they too not emerge incrementally, like the biological or psychological properties that human beings possess? If immaterial person substances of the kind did exist, perhaps the very fact that the sorts of psychological capacities for which those substances are thought to be responsible themselves emerge gradually is good reason for believing that the same is true of the immaterial substances responsible for them.
Secondly, even if the punctualist thesis necessarily follows from dualism, does it follow from dualism that the ‘existential pop’ must occur at some time during the process of conception, and not at any later stage of human development? There is no reason that I can think of for believing that dualism implies this, and therefore that dualism supports the conception threshold of personhood. We must look further, then, for a persuasive argument that the beginning of personhood in humans is contemporaneous with conception.
-  For an inconceivably long stretch of time, none of us did exist as subjects of consciousness, fromwhich fact alone it must be accepted that it is possible for persons to exist at one time and not at another.
-  For a detailed exposition of this problem, see Paul Churchland, Matter and Consciousness (Revisededn, MIT Press) 30—33.
-  For a fuller explanation, see Eric T Olson, The Human Animal: Personal Identity WithoutPsychology (Oxford University Press 1999) and Eric T Olson, ‘ “Personal Identity”, entry in theStanford Encyclopaedia of Philosophy’ http://plato.stanford.edu/entries/identity-personal/ (lastaccessed 10 October 2016).