Human nature is a topic that has occupied human thought for a very long time. Accounts of human nature in ancient, medieval, and early modern philosophy occur throughout many texts from each period.

For the Greeks major topics of concern included happiness, the virtues, knowledge and opinion, body and soul, immortality, rationality, freedom of the will, created being and goodness. Together these topics formed their theory of human nature. This was considered in terms of emanations of divine perfection against which human nature was compared and found to fall short. Human nature was then to live as good a life as possible, but this was easier said than done.

Some texts:

Socrates/Plato - A good man cannot be harmed
Plato - Seventh Letter
Aristotle - Nicomachean Ethics - The golden mean
Aristotle - On The Soul, Book iii, Chapter iv-vii
St. Thomas Aquinas's Commentary - Lectures Seven To Twelve
St. Thomas Aquinas - Commentary On Aristotle's Posterior Analytics, Book Ii, Lecture 20 ‘How The First Principles Of Demonstration Are Known By Us’

The Middle Ages took up where the Greeks left off but considered these factors within a background on monotheism rather than polytheism. The main focus of the discussions were the metaphysical foundations of moral value in terms of standards set by God and exemplified by the divine and divine being such as Jesus. Human nature became thought of as a set of characteristics shared by god and man with even less hope for the individual mortal to measure up to the divine standard.

Some texts:

Boethius - De Hebdomadibus

St. Thomas Aquinas - Commentary On Boethius’s De Hebdomadibus

Saint Anselm’s ontological argument for the existence of God

St. Thomas Aquinas -

On The Principles Of Nature

The Existence Of God

Whether The Existence Of God Is Self-Evident?

Whether It Can Be Demonstrated That God Exists?

Whether God Exists?

Thomas a Kempis - The Imitation of Christ

Literal, metaphorical, physical, spiritual, metaphysical imitation.

The emergence of modern philosophy in the seventeenth century produced conceptual changes that shook the metaphysical foundations set down by the Greeks and developed by the scholastics of the middle ages.

Francis Bacon founded a system of repeatable experimental science in physics and chemistry (previously the subject of alchemy). Rene Descartes set out to do the same for humans and animals. In this light we may judge Descartes to be either the last scholastic or the first modern philosopher, or both!

Most philosophers know Descartes through his writings on metaphysics and epistemology, the Discourse on the Method (1637) and the Meditations on First Philosophy (1641), in which he defends the immateriality of the soul and seeks secure methodological foundations for human enquiry. Humans are thinking things (a res cogitans), spirits or souls, somehow associated with material bodies.

The declaration of the cogito that ‘I think therefore I am’ implies that our true nature is this thinking thing. In contrast, non-human animals are automata. They are not thinking things. Though living beings with nervous systems, they have no true spirit or soul. Their behaviour is explicable wholly in terms of physical mechanisms.

As well as his meditative philosophy, Descartes also had a biological approach to human nature. This is set out in L’Homme, the Treatise on Man brought to an almost complete state by late 1633, before the Discourse or the Meditations. Descartes abandoned plans to publish it along with a work on matter theory and optics that relied on Copernican cosmology, after he heard of the condemnation of Galileo. Indeed it may be argued that it was this repression of experimental science that led Descartes into the armchair speculations that produced the Discourse and the Meditations. L’Homme was published posthumously in 1662 (Latin) and 1664 (French). It was not fully translated into English until 1972.

In L’Homme Descartes sought to eliminate teleology (or purposiveness) from life science by showing that biological matter has no specific intrinsic powers or built-in ends, and that its capacities derive from various complex organisations of ordinary physical particles. So he rejected the ‘vegetative’ and ‘sensitive’ souls of Aristotle and Galen. There is no corresponding dualism between matter and life in his work. Physiological functions differ only in complexity from other mechanical operations. This is underlined by Descartes’ conceit that L’Homme describes only a fictional world of soul-less ‘earthen machines’, self-moving automata analogous to artificial hydraulic engines in ‘the gardens of our kings’.

The bulk of L’Homme is devoted to muscular motion, reflex action, and sensation, all explained by reference to the structure of the nerves, the nature of the flow of animal spirits or nervous fluid through nerves and brain pores, and the current state of the brain.

In the course of this investigation he assigns the pineal gland a unique role, as the link to the soul and the organ with executive control of brain processing. As a unitary and central structure, the pineal gland sways or dances on its supporting network of arteries, and can thus direct the patterned flow of animal spirits through the tubes and traces of the brain. He says ‘The whole brain is nothing but a tissue constituted in a particular way’.

Perceptual representation occurs through the neural transmission of patterns structurally isomorphic with the sensory input. Physical ‘ideas’ are the patterns traced in spirits on the surface of the pineal gland.

However, from dissection Descartes observed that non-human animals also have pineal glands. Thus, he allows representation and indeed sentience even in ‘beast-machines’.

Descartes does not conceive of the body as an object cut off from the environment and responding passively to current stimuli or to the whim of the soul or thinking thing. Instead, in addition to many internal systems of non-linear feedback in the maintenance of bodily homeostasis, he allows for direct and continuous lines of influence between body and world.

All the causal influences on physiological processes act holistically, and even an automaton's behaviour is not wholly pre-programmed, for its physiology changes over time, so that automata with different histories will respond differently.

In general, Descartes accepts the biological phenomena, and seeks mechanical explanations through observation while at the same time insisting that human beings are distinct and separate thinking things through logic.

Given his commitment to the study of all aspects of humanity in terms of these revolutionary scientific methods the key question for Descartes is ‘what must human nature be like in order for science to be possible?’ The answer he gives rests on the idea that by an act of (free) will, humans transcend the physical (the material world) and can, in turn, comprehend nature. The mind must be separate from nature for otherwise it too would be a mere physical mechanism, made of matter, with no possibility of an act of (free) will and thus have no way of understanding nature.

Freedom is then a condition for the development of science, for science relies on acts of syntheses. But, without freewill syntheses are not possible because hypotheses (which are essential for syntheses) require the use of imagination which, in turn, relies on acts of freewill. Hence for Descartes, the possibility of science is bound-up with human freedom.

Moreover, science requires all of the mind's faculties, not only reason and imagination, but memory, and sensation. Memory plays an important role in deductive logic (which for Aristotle and Descartes alike defines scientific method), and imagination is necessary for deduction to produce hypotheses.

Descartes held that non-human animals are automata. Non-human behaviour is explicable wholly in terms of physical mechanisms. He explored the idea of a machine that looked and behaved like a human being, what philosophers call a zombie.

With only seventeenth century theories and technological know-how, he thought two things would unmask such a machine:

• it could not use language creatively but could only produce stereotyped responses, and
• it could not produce appropriate non-verbal behaviour in arbitrarily various situations (see Discourse V).

For Descartes, no machine could behave like a human being. He concluded that explaining distinctively human behaviour required something beyond the physical, namely an immaterial mind, interacting with processes in the brain and the rest of the body.

If Descartes is right, there could not be a world physically like the actual world but lacking such minds. Human bodies would not work properly. If we suddenly lost our minds, our bodies might continue to run on for a while. Our hearts might continue to beat, we might breathe while asleep and digest food. We might even walk or sing in a mindless sort of way (see his Reply to Objections IV). But, without the contribution made by minds, behaviour could not show characteristically human features.

Although Descartes did everything short of spelling out the idea of zombies, the question of their possibility did not arise for him. There could be no zombie world. The nearest thing to this would be a planet of automata whose behaviour was easily recognisable as not fully human.

As we have seen, science in the seventeenth century is not the concrete science as delineated in a textbook today. It refers primarily to deductive logic and the philosophy of scientific knowledge. Much seventeenth century scientific work is taken up with epistemological matters about the relations among reason, intellectual and corporal imagination, freedom, hypotheses, sensation, and the external world.

Later on in the seventeenth and eighteenth centuries scientists began to think that physics was capable of explaining all physical events under physical laws present in nature. It seemed that every physical effect has a physical cause, and that the physical world is ‘closed under causation’. This became a central tenet of the enlightenment whereby everything was open to explanation by reason.

Probably the most enlightenment enlightenment thinker in our area is the French philosopher and doctor, Julien Offray de La Mettrie. First in The Natural History of the Soul, circa. 1742, but most famously in The Man Machine (L’Homme Machine), 1748, La Mettrie argues that human beings are essentially constructed like machines and that humans obey the same principles as machine mechanisms. He represents arch-monism: Everything, including humans, is made from the same physical stuff. Only the ‘leavening’ is different.

The Man Machine provides a classic example of how the ideas of the Enlightenment of human autonomy are interwoven with a technical discourse of perfection, as opposed to the Greek discourse on living the perfect life and the medieval theological discourse of perfection. However, La Mettrie is careful to say "There are as many different minds, different characters, and different customs, as there are different temperaments". This alone is enough to show that La Mettrie does not believe man is entirely a machine, even though he calls him one throughout this book. Man is more complicated than a machine, because he can reason, and he can make decisions, which a simple machine cannot do.

It is worth noting that Descartes was criticised less in the seventeenth and eighteenth centuries for his radical dualism than for the opposing possibility that a physicalist view of the mind (such as ‘the mind is what the brain does’) and humanity could be extracted from Descartes' physiological works. La Mettrie is a prime example of this.

In the nineteenth century the developing science of neurophysiology sought to extend physical explanations to human behaviour. This method of explanation became known as physicalism. While there are many varieties of physicalism, straightforward physicalism holds that human behaviour is explicable in purely physical terms. But if physicalism is true, how can human consciousness and, hence, human nature fit into the account?

The straightforward physicalist response is to insist that it is just a matter of physical processes. However, the phenomena of consciousness are hard to account for in those terms, and some thinkers concluded that something non-physical must be involved. Given the causal closure of the physical, they also concluded that consciousness has no effects on the physical world. On this view human beings are, as T. H. Huxley puts it, ‘conscious automata’. For William James, a founding father of experimental psychology, ‘all physical events, human behaviour included, are explicable in terms of physical processes, and the phenomena of consciousness are causally inert by-products’ (see W. James, Principles of Psychology, 1890, Chapter 5).

Later it became clear that this version of physicalism implied that there could be purely physical organisms exactly like us except for lacking consciousness. And, if it is possible for there to be conscious automata (e.g. if consciousness is epiphenomenal) then zombies are not just conceivable but physically possible (see below).

And so to today. After all these centuries pretty much all we can uncontroversially say is that humans share an observable physical aspect and an unobservable (at least by conventional methods) rational aspect. What is distinctive of human nature is the human view of the world, the view from where I am. It makes sense to ask what it’s like to be a human, to be me.

Now consider a biological organism that shared all our observable physical aspects but had no rational aspect. Such a thing is what philosophers call a ‘zombie’. Zombies are exactly like us in all physical respects but have no conscious experiences. There is by definition nothing it is like to be a zombie. Yet zombies (contra Descartes) behave like us, spend a lot of time discussing their thoughts and feelings, display conscious behaviour, use a rich vocabulary, and may even hold day schools and write books on human nature and the conceivability of zombies.

Such disconcerting thoughts help to make the problem of phenomenal consciousness vivid. They present a special problem for physicalists who believe that our rational, mental life can be explained by, or in terms of, our observable, physical aspects, usually our brains, for there’s no point in producing an account of human nature, in biological or other terms, that simply describes what zombies do. Such a theory would present the evolutionary psychology, the cognitive science, the cognitive neuroscience and the behavioural genetics together with the a priori false claim that this is what it is like to be human.

Though the science may be 100% perfect, the claim to have accounted for human nature, for what it’s like to be a human would be dead wrong. At best what you’d have would be an account of zombie nature. The challenge here is to explain why a so-called biological theory of human nature can ever be more than a theory of zombie nature. There must be a difference, mustn’t there?

But why should scientists or anyone else worry about this? I don’t know anyone who believes that zombies actually exist. But many hold they are at least conceivable, and some that they are logically or metaphysically possible. It is arguable that if zombies are so much as a bare possibility, then physicalism is false, some kind of dualism must be true, and there can be no complete account of human nature that relies on the biological sciences (such as evolutionary psychology, cognitive science, cognitive neuroscience and behavioural genetics) alone.

Zombies also raise epistemological difficulties. They raise a version of the problem of other minds. If we admit the possibility of zombies then how do I know what is a zombie and who isn’t a zombie? Am I addressing an audience of zombies, seemingly appreciative of my every word but in fact non-conscious and oblivious to anything I say or do? How do you know that I am not a zombie, a performing mechanism, delivering wise words and foolish doggerel in the same persuasive manner? And, perhaps shockingly, if either or both of these scenarios were real, would it matter, would it make any difference?

But isn’t this all nonsense? Surely we can’t dismiss biology and genetics as the explanation of human nature by some fairy story about performing zombies? Well, let’s step back a bit and look at the plausibility of such things as zombies. Perhaps the most powerful argument for us to spend useful time on such seemingly unlikely circumstances is the conceivability argument for zombies. This goes as follows:

Versions of it have been advanced by such notable philosophers as Saul Kripke, 1972, Tom Nagel, 1974, David Chalmers 1996, and Joseph Levine, 2001.

Clearly the argument is valid. However, both its premisses are problematic. They are unclear as stated, and controversial even when clarified.

A key question here is how we should understand ‘conceivable’ in this context. We may hope to get out of trouble by an appeal to semantic vagueness. On the other hand, if we are to insist that zombies are not possible then surely the burden of proof lies with us!

The usual assumption is that none of us is actually a zombie, and that zombies cannot exist in our world. The central question, however, is not whether zombies can exist in our world, but whether they, or a whole zombie world (which is sometimes a more appropriate idea to work with), are possible in some broader sense.

The possibility of zombies would seem to pose a problem for evolutionary theory. Why should creatures with conscious inner lives have survived rather than zombie counterparts of those creatures? How could consciousness possibly have an ecological function or evolutionary purpose?

Owen Flanagan and Thomas Polger are two philosophers who have used the apparent possibility of zombies to support the claim that

One response is to suggest that there are as yet undiscovered fundamental biological or psycho-biological laws linking conscious experience to the physical. Such laws would not depend on whether conscious creatures ever happened to evolve. In this case, arguably, evolution poses no special problem (For a version of this response see David Chalmers - The Conscious Mind, OUP, 1996, p. 171).

But who knows what the truth is? At the end of the day, while you may be able to slant the odds in your favour, you pays your evolutionary money and you takes your evolutionary chance!

If you would like to know more about philosophers and zombies see Robert Kirk’s entry in the Stanford Internet Encyclopaedia of Philosophy: Click Here