Imagine a row of 5 Petri dishes. In the first, a single human sperm cell. In the second, a single human egg cell. Third, a fertilized human egg. Fourth, a single cell from the skin of a human finger, and fifth a single human cancer cell. Given an appropriate environment, supplied with all necessary nutrients, but without any other intervention, the 5 cells behave differently: the sperm and egg cells survive for a time but do not multiply, while the skin cell and cancer cell multiply but form only shapeless masses.
The fertilized egg, however, is different. Alone among the five different types of cell, the fertilized egg begins to grow and multiply, not just in a shapeless mass, but in a purposeful pattern. Though that pattern may be hard to recognize at first, it is growing distinct structures, building foundations for what will later become organs and limbs. In short, the fertilized egg follows the normal pattern of human development.
Why does a fertilized egg begin to develop as a human individual when the sperm cell and unfertilized egg do not? Why does a cancer cell grow into a tumor instead of growing its own arms and legs and nervous system? Why do skin cells and bone cells and brain cells never begin to develop as individual organisms? Why don't warts ever "bud off" and become separate people?
If we were extremely dedicated to religious explanations for things, we might suppose that it is all some kind of miracle, that the Good Lord sends angels down to watch over each cell and lead it in the way it should go, and things happen this way just because it's God's will. Or, if we were superstitious and inclined towards a more classical mythology, we might suppose the difference in the behavior of various cells was due to the influence of fairies and gremlins. We could, if we really wanted to, devise explanations that presume there is nothing inherent within the cells themselves that could possibly explain this phenomenon, and that it must be due entirely to outside, supernatural influences.
Of course, everyone knows that the physical factor that makes you a human being is your DNA, right? Well, partly right. The reason we refer to this physical factor as the PHI factor rather than just DNA is because DNA, by itself, is not conclusive: every cell in your body possesses human DNA, but this does not make each separate cell a distinct human individual, nor are your cells likely to suddenly start growing and developing into separate individuals just because they each have human DNA. Though DNA is an essential part of the PHI factor, it is not the only part.
What is the rest, then? We are still learning. We do not yet know all the details of the processes and conditions that cause a single cell to grow and develop into a single, multicellular individual, but we can deduce the presence of a physical, biological factor controlling this development, a factor that is present in the individual from its earliest days, all the way back to the original single fertilized egg.
Where did the child come from? The child is what the baby grew into. The baby grew into a human individual child because the physical human individuality factor needed by the child was already present in the baby. The baby, being a human individual, grew into a child who was a human individual and who, in turn, grew into the adult human individual.
Where did the baby come from? The baby is what the fetus grew into. Is this surprising, miraculous? Not at all. The baby is the fetus, only a little more mature. The baby is an individual organism because the fetus is an individual organism. Note, by the way, we are talking about an individual organism in the biological sense of individual, not in the sense of "person." The fetus exists as a biologically individual organism, and develops as a biologically individual organism, and eventually becomes the baby, which we all recognize as a biologically individual organism.
Thus, by observation, we can deduce that the same PHI factor that was present in the baby was also present in the earlier fetus. Where, then, did the fetus come from? Did some of the mother's cells suddenly and spontaneously decide to quit developing as mere parts of her body and start developing as a biologically individual fetus? Of course not: the fetus came from the embryo. Genetically distinct from the cells of the mother, genetically coherent with each other, the cells in the embryo are busy developing the structures needed for a complete and healthy fetus. Why? Because the embryo possesses, within itself, that physical human individuality factor that tells it to grow and develop as a human individual.
The PHI factor is how the embryo "knows" it's not just a clump of cells like a wart or a bundle of muscle fibers. Because of the PHI factor, the embryo "knows" that, by the time it grows to be a fetus, it is going to have only those bodily resources that it prepared for itself while still an embryo. In other words, we observe that the zygote "knows" it needs to develop into an individual fetus--something we do not observe in random clumps of unrelated cells, or in tumors, or in individual organs within a larger individual. Therefore we deduce that the zygote, too, possesses the PHI factor.
The same reasoning works backwards from embryo to zygote, and from zygote to fertilized egg. At each stage of physical development, the organism works hard to prepare the physiological structures that will be needed by the next stage. We can deduce the presence of the PHI factor throughout the development of the individual by the very fact that it "knows" somehow that it has to grow from fertilized egg to zygote, zygote to embryo, embryo to fetus, fetus to baby, and so on. This process of individual development is unique to individual organisms; cancer cells and isolated skin cells, for instance, never develop into individual human zygotes or embryos, and the sperm and unfertilized egg never grow into any kind of multicellular organism at all.
There is no evidence of any such thing happening, and indeed two things argue against this possibility. One is, of course, the success of in vitro fertilization, i.e. "test tube babies." There are a certain number of people alive today who were originally conceived by putting human egg cells and sperm cells into the same container and hoping that fertilization takes place. When the attempt is successful, the cells are later seen to be developing into individual zygotes. Since the process of individual development begins before the developing individual is even placed inside the mother's body, it is unlikely that anything from the mother's body is what causes the organism to begin developing as an individual.
The other factor arguing against this possibility (as if the above were not enough) is the pattern of evolutionary development. The development of distinct, individual, multicellular organisms was a very early divergence in the evolutionary tree of life, dating back to the very simplest of early organisms. One would suppose, then, that whatever the physical factor is that makes a group of cells into an individual organism rather than a mere colony of related cells, that factor would be a very fundamental factor, and would be present in most, not to say all, of the higher life forms. Comparative anatomy and physiology ought to be a reliable way, therefore, to learn something about this physical individuality factor.
One of the first things we notice, then, is the large number of species that reproduce by laying eggs. Mammals, which appear fairly late on the evolutionary tree, are almost the only creatures whose young do not develop in complete isolation from the mother, encased in individual eggs, yet this does not prevent the lower forms of life from developing as individual organisms. Thus, it is plain that the physical factor that makes an organism an individual organism is a physical factor that is inherent within the organism itself, and is not a factor that is imposed on the organism from outside (i.e. from the mother). If this is true for the lower life forms, it is unlikely that the higher forms would need anything more complicated just to be biologically individualized organisms.
Certainly not. Abnormalities do not prove that the normal condition does not exist. Does the existence of an amputee disprove the claim that humans normally have two arms and two legs?
What these abnormalities do prove is that the factor that makes us all human individuals is a purely natural, physical factor. Human individuality is not miraculously produced by some infallible divine intelligence, but is present as a normal, natural factor within the body of the individual. Thus, like any other physical structure or process within the body, it is subject to damage and malfunction. Naturally, it cannot malfunction if it does not exist, and therefore we can be sure that this physical human individuality factor does exist. If it can be damaged, it is real.
The PHI factor, by contrast, is immediately observable through the impact it has on the individual's development. Almost from the moment the single egg develops a complete PHI factor, through fertilization, that factor begins driving and controlling the growth and development of the individual organism. This same individuality factor (minus the "human" element, of course) can be observed in other species as well, and can be seen to behave consistently across a vast array of lower and higher life forms. Thus, it is not too much to say that the physical and biological individuality factor is at least as well-established (though our understanding is not always as detailed) as any other observed biological fact.
This implies that the PHI factor is far superior to "personhood" as an objective and egalitarian standard on which to base protection of human rights. Personhood is purely a subjective construct, and is subject to whatever personal opinions and prejudices are shared by the majority. So long as human rights are based on personhood, there will be no protection for those human individuals who are viewed as being in some sense inferior or inadequate in the opinions of the majority. Only by basing protection of human rights on a scientifically observable and verifiable standard like the PHI factor can we guarantee equal protection of human rights for every individual member of the human species.