inevitably fail in others, revealing, that they acted not from knowledge but only from the disposition of their organs (ibid.).
Because of this they lack reason, as reason is a "universal tool" that serves in all circumstances.
Exactly the same two points can be applied to distinguish humans from other animals, claims Descartes. Firstly, animals cannot use language and all humans can do this:
... there are no men so dull and stupid, not even lunatics, that they cannot arrange various words and form a sentence to make their thoughts understood; but no other animal ... can do the like. (ibid.).
This is intended to show that animals have a complete lack of reason and not merely a lesser degree of it. Even the best monkeys or parrots are not comparable in this respect to "one of the stupidest children or at least a child with a diseased brain" which shows that their "souls" are "wholly different in nature from ours".
Secondly, animals display the same limitations as machines:
... although several brutes exhibit more skill than we in some of their actions, they show none at all in many other circumstances; so their excelling us is no proof that they have a mind, for in that case they would have a better one than any of us and would excel us all round; it rather shows that they have none... (ibid.)
So the ability to reason is what separates us from other animals. In fact, for Descartes, the error of believing that animals have reason and consequently minds is second only to the error of denying the existence of God as being likely to turn "weak characters from the strait way of virtue".
In showing how much we differ from other animals, Descartes also gives a further indication of what characteristics all humans share. As was remarked earlier, Descartes seemed to exclude "barbarians and savages'" from those who could reason. However, Descartes says that animals do not have the abilities of "the dull and stupid", "lunatics", "the stupidest children" and even a child with a "diseased brain". It would seem then that the "barbarians and savages", despite their lack of culture, would be allowed to share in reason.
So, for Descartes, reason is a vital characteristic. The ability to reason is the feature of a mind, which is a different substance from matter, and is the thing that allows our immortality. The ability to reason is, then, an essential human characteristic, not shared by other animals. This claim is, however, disputed by Hume.
Hume displaces reason from the position that those such as Descartes would give it. Throughout A Treatise of Human Nature, Hume shows again and again that reason is insufficient: for example, reason does not provide us with an explanation of cause and effect; nor with an argument for the existence of the external world; nor an account of personal identi1:)r. Reason is found in varying degrees in different people: in a long note in the Enquiry Concerning Human Understanding, Hume lists some of these, such as differences in memory, attention and observation, the ability to carry out long chains of argument without confusion, biases of prejudice and education, and so on (Hume, 1748). Reason is found in animals to be much the same as it is in humans:
... and no truth appears to me to be more evident, than that beasts are endow'dwith thought and reason as well as man. (Hume, 1739).
When a dog avoids fire and strangers, but "caresses his master", for example, the dog relies on its senses and memory and draws inferences from experience in the same way that we do (op cit.). So if reason is not central to human nature, what is?
To understand Hume'sviews it is important to see what he means by the "attempt to introduce the experimental method of reasoning into moral subjects" that is in the title of the Treatise. Hume intends to give a naturalistic account of humans, that is to treat the study of humans in the same way that the natural sciences treat their subjects. An example of this sort of procedure is supplied by Hume's treatment of free will and determinism. Hume is a determinist in as much as he believes human behaviour to be predictable, and this predictability is of the same kind as the predictability of any natural phenomena. If a traveller reported that in a country of the same northern latitude as ours the fruit ripened in the winter and decayed in the summer, that is, in the opposite way to which it occurs in this country, they would be believed as much as if they had reported people of the same character as those in Plato's Republic or Hobbes'Leviathan (Hume, 1739). A prisoner discovers the impossibility of escape as much from the obstinacy of the jailer as from the stones and bars of the cell, and foresees death as much from the "constancy and fidelity" of the guards as from foreseeing their head being cut off from their body (op. cit.). So in the same way that we learn any facts about the world - from observation - we will have to learn the facts about human nature in the same way.
When discussing ethics, Hume concludes that there are no moral truths to be observed in the world and that reason cannot supply a basis for ethics either. Morality is a matter of feeling, or as Hume would say, sentiment. In looking for the wrongness of an action:
The vice entirely escapes you, as long as you consider the object. You never can find it, till you turn your reflection into your own breast, and find a sentiment of disapprobation, which arises in you, towards the object (Hume, 1740).
But if morality is simply a matter of feeling, and reason plays no role other than informing us of the existence of the objects of our desires and the best means to attain them, then why don'twe just do anything that we feel like? Hume is famously quoted as saying
'Tis not contrary to reason to prefer the destruction of the whole world to (the scratching of my finger (Hume, 1739).