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[55] I have explained all these things in enough detail in the treatise which I originally intended to publish. [n.1] After that I showed:

• how the nerves and the muscles of the human body must be constructed, so that the animals spirits have the power to move the parts of the body when they are in it — for example, it has been observed that when people’s heads have been cut off, they continue to move for a while and bite the earth, even though the soul is no longer present;
  
  
• what changes must take place in the brain to cause waking, sleeping, and dreams;
  
  
• how light, sounds, smells, taste, warmth, and all the other qualities of external objects can impress various ideas on the brain through the agency of the senses;
  
  
• how hunger, thirst and other internal passions can also transmit their ideas to it;
  
  
• what part of the brain must be taken as the ‘common sense,’ [n.2] where these ideas are received;
  
  
• what part of the brain must be taken as the memory, which preserves these ideas;
  
  
• what part of the brain must be taken as the imagination, which can change these ideas in various ways, and make new ones out of them, and by the same means distribute the animal spirits among the muscles, and make the parts of the body move in as many different ways, and as appropriately to the objects which present themselves to its senses, and to the internal passions which are within it, as ours can move without being prompted by the will.

This will not seem at all strange to those who know how many different ‘automata’ (machines which move) can be made by human skill. [56] But these human artefacts have far fewer components than the vast quantity of bones, muscles, nerves, arteries, veins, and all the other parts which make up the body of each animal. So these animal bodies can be considered as machines; and since they have been made by the hands of God, they are incomparably better constructed, and have internal movements which are more marvellous than any which can be invented by humans.

At this point, I went out of my way to emphasise that, if there were any machines which had the organs and external shape of a monkey, or some other animal which lacked reason, we would have no way of telling that they were not of completely the same nature as these animals. By contrast, if there were any such machines which resembled our bodies, and imitated as many of our actions as their nature allowed them to, we would still have two infallible ways of detecting that, despite this, they were not genuine human beings.

The first is that they would never be able to put words or other signs together, as we do for revealing our thoughts to others. It’s easy to imagine a machine which can utter words — and even one which can utter certain words which are appropriate to physical events which cause changes in its internal parts. For example, if it is touched in one place, it asks what you want to say to it; and if it is touched in another place, it screams that you are hurting it; and so on. What it cannot do is to combine words in different ways, so as to respond to the meaning of everything that is said in its presence — [57] as even the most dim-witted humans can do.

The second is that, even though they might be able to do many things as well as, or perhaps even better than we can, there are other things which they could not possibly do at all. These deficiencies would reveal that they were not acting on knowledge, but simply as the result of the physical state of their components. This is because reason is a universal faculty, which can be applied to any situation, whereas the components of a machine are restricted to a particular reaction to a particular state. The consequence is that a machine, by its very nature, cannot contain enough diversity to react to the contingencies of life in the way that we do, guided by reason.

So these two factors enable us, not only to discriminate between humans and other animals, but to have a scientific knowledge of what their specific difference consists in. It is a distinctive characteristic of all humans, however dim-witted, stupid, or even plain mad, that they are capable of putting different words together into a discourse through which they can make their thoughts understood. No other animal, however perfect or fortunate, can achieve anything like this. And it is not because other animals lack the required organs. Magpies and parrots can utter words just as we do, but they cannot enter into conversations as we do — that is to say, they cannot give evidence that they are thinking what they are saying. By contrast, humans who are born deaf and dumb [58] are even worse off than some animals as far as the organs of speech are concerned, but they usually invent their own signs for making themselves understood to people close to them who have time to learn their language.

This is evidence, not merely that animals have less reason than humans, but that they have no reason at all. We know that very little reason is needed to know how to talk. We also observe that it is as true of animals which belong to the same species as it is of humans, that some are easier to train than others. So it is only because the souls of animals are of a completely different nature from ours, that we cannot believe that the supreme examples of the species of monkey or parrot could be equal in this respect to the stupidest of human infants, or at least those suffering from brain damage.

Moreover, words must not be confused with the natural movements which reveal feelings in animals, and which can be imitated perfectly by machines. Nor should we believe (as some ancient philosophers did) that animals speak, but without our understanding their language. If this were true, they would be able to make themselves understood to us as well as to members of the same species, since they have many bodily organs which correspond to ours.

It is also a remarkable fact that, although there are many animals which display more ingenuity than us in a few of their actions, you will always observe that the same animals display none at all in many others. Consequently, the fact that they do some things better than us doesn’t prove that they have any intelligence. If so, this would mean that they were more intelligent than any of us, and would beat us at everything else. [59] Rather it proves that they have no intelligence at all, and that their actions are naturally determined by the state of their bodily organs. They are just like clocks made of nothing but wheels and springs, which can count the hours and measure time more accurately than we can, despite all our wisdom.

After that, I described the rational soul, [n.3] and showed that it can in no way be derived from the power of matter, unlike the other things I discussed, but that it must have been specially created. Again, it is not enough for it to dwell in the soul like a captain [n.4] in his ship. That would perhaps be sufficient if all it did was to move the parts of the body; but it must be joined and united more closely to the body in order for it also to have sensations and appetites like ours, and thus to constitute a genuine human being.

However, at this point I dwelt for some time on the topic of the soul, because it is one of the most important. Apart from the error of denying the existence of God (of which I think I have given a sufficient refutation above), there is no error which does more to distance people of weak intelligence from the straight path of virtue, than to imagine that the souls of animals are of the same nature as ours, and therefore that we have nothing to fear or hope for after this life, any more than flies or ants. Whereas, as long as we know how very different they are, we have a much better understanding of the reasons which prove that our soul is of a nature which is entirely independent of the body, and therefore that it is not subject to death along with it. [60] Furthermore, since we cannot see any other causes which might destroy it, we are naturally led to judge that it is immortal.

Go to Index to the Discourse on the Method