The question whether or not human life begins at conception is strictly speaking a philosophical question. This does not mean, as is often assumed, that the question cannot be answered with certainty and that it is only a matter of opinion whether the embryo is a human being or not. What it means is that the question cannot be resolved by sensation or investigation, but by reason. For it is quite possible for two embryologists who have the same data to draw two contradictory conclusions regarding the status of the developing embryo. The differences are not due to the scientific data, if the data is the same in both cases. Rather, their divergent perspectives are due to their reasoning. Nevertheless, philosophy depends upon science in order to be able to resolve the question through reason. The science of human embryology provides the data without which a philosopher cannot begin to answer the question about the status of the embryo and the beginning of human life.
In order to resolve this question, I'd like to begin with some basic philosophical principles. We know from our own experience that all knowledge begins in sense perception. But to say this is not to say that it ends in perception. Indeed, knowledge is much more than sense perception, for intelligence is the ability to apprehend the intelligible structures of things. What is sensible, such as the temperature of an ice cube or the color of a surface, is not the same as what is intelligible, such as the nature of a triangle or the essence of a dog. But human persons do not grasp the intelligible natures of things immediately, but only indirectly. It is only through its activity that we come to understand the nature of a thing; for activity discloses to us precisely what a thing is. The reason for this is that a thing acts according to the powers of its nature. For example, dogs bark, ducks quack, plants grow and reproduce, birds fly, and human persons reason and laugh, etc. Thus, activity precedes essence in the order of human knowing, while essence precedes activity in the order of being for a thing cannot act unless it first exists as a certain kind of thing.
To understand this better, consider the game of chess. If someone were to ask about the pieces, for example the Queen, the Bishop, or the Rook, that is, if he were to ask what they were, it would not be enough simply to point to them on a board. To really understand the pieces, we need to know what they do, that is, how they move. The Bishop moves diagonally, the Pawns move forward one space at a time and capture diagonally, the King moves one space at a time in any direction, and the Queen moves in any direction as many spaces as the player desires, etc. In other words, we know each piece by its function or activity.
A King has the potential to move one space at a time in any direction, but circumstances might be such that it cannot move at all, for example, if there are no empty spaces next to it, as is the case at the beginning of a game. But the King is still a King, that is, it still has the power to move in any direction, one space at a time, circumstances permitting. And should a Pawn successfully make it to the 8th rank, it turns into a Queen, just as a checker piece that makes it to the 8th rank is crowned a King. It often happens that the player still has his Queen on the board when his Pawn becomes a second Queen. What usually happens in such cases is that the player will retrieve a captured Rook, and that piece becomes a Queen. It does not look like a Queen, but it is a Queen because it functions like a Queen, that is, it has the power to move in any direction and for as many spaces as the player wishes. Despite its appearance, it is more than a Rook, and we know this only by observing how it functions. It functions like a Queen, because it has the potentiality of a Queen, because it is a Queen.
Imagine that we've only been given the function of each piece, that is, a description of how each piece can move, without being told what those pieces look like. For example, we've been told that a Bishop can only move diagonally, but we have not been told what the Bishop looks like in order to identify it. It is up to us observe a game in action and correctly match each name to its corresponding piece on the board. A game has just begun, and after a few opening moves, a piece (the Queen) is moved only one space forward. At this point all we know is that it is behaving like a Pawn. The Queen, though, is more than a Pawn, even though at this point in the game it has only moved like a Pawn would move. Likewise for the next move; the second player moves the Queen three spaces diagonally, thus moving like a Bishop. At this point it only appears to move as a Bishop would. It actually has the power to move in any direction, that is, horizontally, forwards, backwards, etc. Hence, it is more than a Bishop, even though we may not know this for certain at this stage of the game. It is only by observing the pieces as the game progresses that we will discover that the Queen is more than a Pawn, more than a Bishop, and more than a Rook, because eventually we see that it functions in a way that these pieces do not and cannot.
So too in the case of the developing embryo. We can come to understand what an embryo is by observing its behavior. Now any developing organism, in the initial stages of its existence, does not and cannot behave as it would fully developed. The reason is that development implies that the organism is in some ways different initially than subsequently. But developing organisms become what they are. They do not become what they are not. A single cell zygote is actually, not potentially, a human person. Spermatozoa do not become human persons. Oocytes do not become human persons. But the oocyte becomes something else entirely when touched by a sperm. What results is something entirely new, moving in an entirely new direction. What this new something is can be determined by observing its new behavior.
When a sperm reaches the protective shell surrounding the oocyte, called the zona pellucida, it binds with a glyco-protein sperm receptor molecule. The acrosome, a vesicle that caps the head of the sperm, releases degradative enzymes that allow the sperm to penetrate the zona pellucida. When the sperm successfully penetrates the zona pellucida, the cell membranes of the two cells fuse. This immediately causes thousands of small cortical granules located just beneath the cell membrane of the oocyte to release their contents into the space between the oocyte and the zona pellucida. These substances interact with the zona pellucida, altering the sperm receptor molecules, causing the zona to become impenetrable by additional sperm.
The fusion of the cell membrane of the sperm with the membrane of the oocyte causes the oocyte to resume meiosis. The embryo now contains a diploid complement of chromosomes and a 2N quantity of DNA. Hence, the fertilized oocyte is called a zygote.1
A zygote is neither a sperm nor an oocyte. What it is becomes evident as we observe its behavior and watch it unfold, just as we come to understand the nature of anything by observing its activity.
Just before implantation on the lining of the uterus, the cells of the embryoblast begin to differentiate into two layers called the epiblast (primary ectoderm) and an internal layer called the hypoblast (primary endoderm). The trophoblast at the embryonic pole of the blastocyst proliferates to form the syncytiotrophoblast, which erodes the lining of the uterus, creating an implantation site for the blastocyst and drawing the blastocyst into the uterine wall.
In the third week of development, the bilaminar or two-layered germ disc is converted into a trilaminar or three-layered germ disc (endoderm, mesoderm, and ectoderm). The cells of the endoderm will form the lining of the lungs, tongue, tonsils, digestive tract, and more. The cells of the mesoderm will form muscles, bones, lymphatic tissue, heart, lungs, and more. The ectoderm will form the skin, nails, hair, the lens of the eye, nose and mouth, the parts of the nervous system, and more.
Why does the single cell zygote develop into a two cell embryo, and then a morula (4 days), and then a blastocyst (5-7 days), then a bilaminar germ disc, then a trilaminar germ disc, etc? After a time we know that it always had the power to become a blastocyst, that is, right from fertilization, yet it needed time to become a blastocyst, or a germ disc, because it takes time for cells to divide. If it did not have the power to become a blastocyst, for example, it would not have done so.
What often happens in a discussion on the nature of the embryo is that the term "potentiality" or "potential" is used equivocally. Terms that are equivocal are called one thing, but they have either a slightly or entirely different meaning, as "bark" can be said of the tough outer covering of a tree, as well as the harsh sound of a dog and/or a small sailing vessel. In the case of "potential" in the context of this discussion, there are two modes of having potentiality that are slightly different. Failing to distinguish the two leads to equivocation.
Potentiality means ability or capability. All potentiality has reference to actuality, but there are two kinds of actuality: first act and second act or activity, and corresponding to these two acts are two different potentialities. Firstly, there is the potentiality to become a different entity. I am actually a living human thing (first act), but I have the potentiality to become a corpse (a collection of non-living substances). I become a corpse by ceasing to be what I actually am, namely a living human kind of thing. A sperm becomes a zygote by ceasing to be what it is. So too, an oocyte becomes a zygote by ceasing to be what it is.
Activity (second act) manifests power or potentiality, but this potentiality has reference to activity, not to an entirely new entity (first act). The actualization of these potentialities to activity does not entail that the thing cease to be what it is. Rather, it is through these activities that it comes to be itself most fully. I am actually a human person with a host of potentialities (to activity) that belong to the nature of the human person, not to a corpse, for example the potentiality to actually see (second act or activity), to hear (activity), to run (activity), to think (activity), etc. These latter potentialities are powers to further activity. Only actual human persons actually have the power or potentiality to see, hear, and laugh, etc. We know through observation that sperms do not, because they never eventually come to see, hear, and laugh. But the embryo does eventually come to see, hear, kick, suck its thumb, and cry. It functions like a human person, because it has the potentialities of a human person, because it is a human person. The single or two cell zygote or bilaminar germ disc cannot realize all its initial potentialities until parts of itself undergo further development and maturation, just as the newborn infant cannot realize many of its potentialities until its parts undergo further development. But it is not potentially a human person, but is actually a developing human person with a host of potentialities that the sperm and oocyte, considered in themselves, do not have. The potentialities belonging to the zygote require material organs as conditions for their realization, organs that are already in the process of being developed.
A sperm, on the other hand, is not actually a zygote, but potentially a zygote. One can continue and assert that the zygote is not actually a blastocyst, but potentially a blastocyst, and that a blastocyst is not actually a fetus, but potentially a fetus, etc. This is true, but there is an equivocation here, subtle and difficult to detect, that confuses the two different kinds of potentiality. The sperm is actually a sperm, and it does not develop into a zygote. It is a haploid with a relatively short life span. So too, oocytes do not undergo cleavage and develop into bilaminar and trilaminar germ discs. The sperm (or oocyte) and zygote are two entirely different entities, neither one having the same potentialities and thus the same behavior. The zygote and the blastocyst, on the contrary, are the same entity, the latter being more of the former. There is a continuum between the zygote, the morula, the blastocyst, etc. There is no continuum between the sperm and the zygote, or the oocyte and the zygote.
In order to deal with the question whether the embryo is a part or a whole, let's consider once again the analogy of the chessboard, but this time from a different angle. The Queen is a part of the game of chess, not a whole unto itself. The game as a whole is understood through its end (to achieve checkmate). Each piece has a function that, ultimately, can only be understood within the context of the entire game. Take a Queen off of a chessboard and put it onto a checkerboard in order to replace a missing checker piece and it is no longer a Queen. In the context of the game of checkers, it does not have the power to move in any direction and as many spaces as we wish to move it. It is nothing more than a single checker piece. In fact, it has less power than a crowned King. What was once a Queen is now a part of a different game, having an entirely different function within the context of the new game.
Part and whole thus have a relationship to one another. Parts have a place within the whole, and their function can only be understood in relation to the whole, because they function for its sake. A living whole tends to preserve its own integrity, and its parts function for the sake of that integrity. The function of a part of a living whole, such as the heart or liver, is ultimately to contribute in some way to the integrity of the whole organism.
In this light we can say that the embryo is not a part of the mother. For it does not function as a part. It does not function for the sake of her physical integrity. Rather, the embryo is a whole unto itself. Indeed, it has a certain likeness to the various parts of the mother. For example, the embryo depends upon the mother, as parts depend upon the whole of which they are a part. In addition, the embryo is inside the mother, as the mother's liver and kidney are inside of her. But the child's dependency on the mother is only temporary, and one cannot conclude that because parts are inside the whole, and the embryo is inside the mother, the embryo is a part of the mother any more than one can conclude that because all turkeys are born from eggs and all chickens are born from eggs, all turkeys are chickens. Such logic is invalid and involves an undistributed middle term.
A living part of a living organism is alive not by virtue of a distinct life principle, but by virtue of the life principle of the whole. My finger is alive because I am alive. Sever any part from the whole and it ceases to be what it was2. The life of the embryo is not the life of the mother. The embryo, unlike any part of the mother, has a life unto itself. It is a whole unto itself with its own parts, working in harmony for the sake of its own integrity, depending on the mother for matter, but not in order to be what it is. The definitive proof of that is the birth of the child.
1 William J. Larsen. Human Embryology (3rd edition). New York. Churchill Livingstone. 2001. p. 18. [Back]
2 There are exceptions to this rule. Many plants can be cloned with cuttings. This is done when a portion of the stem with leaves is severed from the parent and placed in a suitable rooting medium for that species of plant (moist sand, a mixture of peat moss and soil, or water). The stem is later transplanted to soil after roots have developed from the cut end of the stem. Some plant life and the simplest of animal life forms (such as worms) are so lacking in complexity and differentiated organization that all the parts share rather closely in the operation of the whole. But this is not so for more complex life forms. [Back]