Before we can talk about humans, and what a human is, we must begin by asking about whether a human-being is one being. That is, is mankind a unity, a single substance, or compilation of substances. “For, unless a man is one being, there is no point in asking what kind of being he is.” Once we have determined that man is one being we must ask what man does. “For it is only through the activities of things that we can discover their nature.” Klubertanz continues explaining the method of learning about man by pointing out that, “The point of departure cannot be the nature, essence or being of man. We can only begin with man’s sensible qualities and with his activities—in other words, with the data of external observation and introspection.”
Most people will recognize this procedure as the inductive method, and all Naturalists, of all sorts, will agree that this is the way to study man. “Induction begins with perception of particulars, which in turn gives rise to retention of perceptual contents, or memory…Craft and science arise from experience, which is a unified set of such memories…Getting from particulars to universals, therefore, is a largely non-inferential and epistemologically unproblematic process…The transition from experience to craft or science results in the grasp of universals.” As we noted in a previous post, Aristotle, and Aquinas, recognized that humans learn by observation.
For most people, to even ask if a human-being is essentially a unit – one, it seems like we are asking about the obvious. You see a human in front of you, it is one. However, there are at least two different ways of viewing humankind that brings into question its essential unity. First of all, scientists, when they look at a human-being, confront this naïve observation with the fact that this so called unity, when subjected to in-depth scrutiny, turns out to be a multiplicity. A human-being is not one, but many. The human has many organs that can be removed or replaced; even these organs are made up of smaller, and smaller, entities. The more precise you get, the less unity you find. So, before we can even study human-kind, we need to determine that a human-being is one, and more importantly that this is an intrinsic, and substantial unity.
Secondly, there is a philosophical theory about the composition of humankind, called Substance Dualism, which sees man as the correlation of two different, even opposite, substances. This theory has been represented by some of the greatest philosophers, and theologians known to man, including Plato, Augustine, Rene Descartes, etc. In this theory, the two opposing substances are referred to as "soul" or "spirit" and "body". The human person is often identified with the immaterial part, which is somehow connected to the body. In this theory man is not essentially one, but rather is an accidental unity.
Let us consider what it means to talk about a unity, what kind of unities there are, and what kind of unity the human person is.
An intrinsic, substantial unity is one in which: (1) the parts are united by nature, that is, “they are ordered to each other and to the whole.” (2) The whole is a complete substance composed of parts that are substantial, but not, individually, complete substances themselves.
As mentioned above, the individual parts of a human-being, from greatest to smallest, in view of the fact that they can be removed and replaced, without removing the human persons identity, seems to imply that a human is, at best, an intrinsic, but accidental unity. A human can be repaired by a doctor in much the same way that a car is repaired by a mechanic. Is a human, then, just an accidental unity, like a car, I-pod, or lap-top, or, is a human a substantial unity?
There are a number of good reasons to think that a human-being is a substantial unity. First of all, Herbert McCabe notes that there is a significant difference between an accidental unity and a substantial unity. He would say that a living thing, such as a human, is a natural unit, and that a car, a non-living assemblage of parts, though its parts work together, is only a quasi-unit. “Let me repeat that an organic structure is one in which the whole is prior to the parts, so that we give an account of the parts by reference to the whole and not vice versa.” The opposite, accounting for the whole by reference to the parts, is how we account for a quasi-unit, a non-living thing.
A second reason is the self-awareness of the human-being through change. Even if a human is able to keep all of his/her organs and extremities for their entire life (never needing any replacement surgery) they are still changing. The human child by the time they have reached adulthood will have not only changed in size, but all of the atoms of which he is composed will have been replaced an innumerable amount of times. Yet, throughout that change, the human will be constantly conscious that their thoughts, memories and knowledge are their own. This is called by Kant, the unity of apperception, and constitutes a proof, not only of the substantial unity of man through change, but also of a fatal inadequacy of the philosophical position known as Naturalism.
A third proof, which correlates with the self-awareness of the human through change, is the self-awareness and self-identity of the human over time. Though time passes, the human is always aware of his self-identity, that his memories are his own, and of his plans for the future.
There are other proofs, but it is not necessary to list all of them. The proofs mentioned above show us that a human being is an intrinsic, substantial, unity. In a substantial unity, the parts must be substantial, but not individual substances. “In order that one substantial whole may be actually one substantial being; they cannot be complete beings in themselves. Two substantial acts make two beings; consequently, there can be only one substantial act in a being that is substantially one.” Further consideration of human actions will demonstrate that a human-being is a substantial unity.
George P. Klubertanz, The Philosophy of Human Nature (New York: Appleton-Century-Crofts, 1953), 8.
Ibid. Such a claim may at first sight seem confusing, because, it seems that if it is true then we cannot know inanimate things. There are two responses to this problem. First of all, inasmuch as a thing exists, it is properly in act. Therefore, insomuch as it is in act, its nature can be known. Second, inanimate things do not properly have a nature, and so, cannot properly be called substances. Inanimate things are quasi-substances.
Ibid. Introspection is essentially reflection, it is what we might call, observing one’s own inner workings. We cannot observe other people’s inner thoughts. The inner workings of our minds are our own, which is another problem that Naturalism has to deal with, and is why it seems more than likely that Chalmer’s optimism, about one day being able to observe the thoughts of another person, is misplaced. Klubertanz says, “However, we cannot directly see or hear or touch an act of knowledge performed by someone else. We can only observe external activities of others and interpret what we see in accord with our own internal experience.” (Ibid., 61-2.)
C. D. C. Reeve, Substantial Knowledge: Aristotle’s Metaphysics (Indianapolis, IN: Hackett Publishing, 2000), 18-19.
Klubertanz, The Philosophy of Human Nature, 13. This is what it means to have intrinsic unity.
This is what it means to be a substantial unity. A substance, according Clarke, is “the same essence considered as that which renders a being apt to exist in itself and not in another – i.e., not as a part of any other being—and which therefore functions as the principle of unity holding together all its various accidental attributes and the abiding principle of its self-identity down through all its accidental changes across time.” (W. Norris Clarke, The One and the Many: A Contemporary Thomistic Metaphysics (2001; repr., Notre Dame, IN: University of Notre Dame, 2007), 159.)
An accidental unity is one in which the parts are only united by a common goal, activity, or purpose.
 Herbert McCabe, On Aquinas, ed. Brian Davies (London : Continuum, 2008), 8-12.
There may, perhaps, be exceptions to this claim in certain cases; however, these exceptions are not the norm for human experience, which is why they are exceptions. Such exceptions cannot be used to nullify this claim. The exceptions are most likely cases of malfunctioning parts, the introduction of disease, or the tampering of other humans. Once a norm is established exceptions are easier to explain.
Cf. Robert Edward Brennan, Thomistic Psychology: a Philosophical Analysis of the Nature of Man (1941; repr., New York: The Macmillan Company, 1946), 72. Klubertanz, The Philosophy of Human Nature, 19-34.
Klubertanz, The Philosophy of Human Nature, 17.