PhilosophersRené Descartes

René Descartes

Doubt everything to find what survives doubt.

Rationalism1596 – 1650 FranceRationalistDualistTheistMathematician
Discourse on Method, V, 1637; letters to Newcastle (1646) and More (1649)

Beasts are machines: they have no thought

What it claims

Beasts have no thought: it is nature that acts in them, according to the arrangement of their organs, as in a machine.

Why

If an animal had thoughts, it would show them through words or signs; none ever has, even the best endowed.

Valid with an implicit premise

For Descartes, an animal's body is a machine, a very fine mechanism like a clock. The question is whether the animal also thinks. Descartes says no, for two reasons. First, no animal has ever used words or signs to say what it thinks, whereas every man does, even a deaf-mute: this is not for lack of organs, since parrots utter words. Second, what animals do that is marvellous, sometimes better than we do, is explained by the arrangement of their organs, like a clock that tells the time more exactly than we can. One nuance matters a great deal, and Descartes stresses it in his 1649 letter to Henry More: he denies thought, not life, nor sensation in the sense of a movement of the body (sensation "in so far as it depends on a bodily organ"). He also says that one cannot prove that beasts do not think: he only judges that this is the most probable view. Now for him, to think means everything one is conscious of (Principles I, 9). In the question "Do animals matter as much as we do?", this argument supports the "Humans first" camp: if beasts do not think, we owe them nothing as we owe it to thinking beings. Scholars discuss what exactly Descartes grants them (our summary).

P1 · implicit premiseIf a beast had thoughts, they would show: either in words or signs to tell them, or in actions that only thought could explain. Dogs and other animals express their passions to us: they would just as well express their thoughts, if they had any. Apart from speech and signs made to the point, no outward action can assure us that a body is not merely a machine.
P2No beast, however well endowed, has ever used words or signs to make thoughts known, whereas no man is incapable of it, not even the dullest, nor the deaf and dumb who invent their own signs. It is not for lack of organs: magpies and parrots utter words, but without showing that they mean what they say.
P3What beasts do that is remarkable, even better than we do, does not prove they have a mind: otherwise they would have more than we do and would do better at everything. It shows rather that nature acts in them according to the arrangement of their organs, as in a clock, which measures time more exactly than we do. Swallows returning in spring, and bees, act in this way like clocks.
So beasts have no thought: it is nature that acts in them, according to the arrangement of their organs, as in a machine.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P1, the computer finds a case where the beast does not speak and acts like a machine, and yet it thinks: a silent thought, with no trace we can read. The argument only shows the absence of thought if one grants that every thought shows itself.
The argument is valid. P1 is the hidden premise: if a beast thought, it would show, in language or in acts that mechanism does not explain. P2 says that no language is found in beasts. P3 says that no act is found that goes beyond mechanism. So they do not think (it is a disjunctive argument: if either trace were present, one could conclude thought). Whoever rejects the conclusion can challenge P1 (a thought may exist without showing itself in signs we recognise), P2 (animals communicate, but we do not understand their signs) or P3 (some acts are better explained by thought). Descartes himself, in his letter to More, presents the conclusion as the most probable, not as proved.
Analogy. A child with a toy that says "mummy" when its belly is pressed does not think the toy thinks: it has seen that it always repeats the same thing, whatever is said to it. Descartes says the same of the parrot and the dog: they have signals tied to their wants, but never a speech invented to say a new idea.

Objections

5

Each objection is a conversation: click to see who replies and where the debate stands.

Michel de Montaigne 1580 · attacks premise 2

When he plays with his cat, who knows whether she is not passing time with him more than he does with her? We amuse one another with our monkey tricks: if he has his hour to begin or to refuse, she has hers. So we do not know what beasts say to each other or to us.

Descartes replies
see the argument
Descartes 1646 · Descartes (letter to Newcastle, which names Montaigne)
One cannot say that beasts speak among themselves and that we merely do not understand them: for, as dogs and some other animals express their passions to us, they would express their thoughts to us just as well, if they had any.
Where the debate stands (our summary)Disputed. Descartes holds that if beasts had thoughts, we would see them expressed as their passions are. Montaigne replies, in advance, that our ignorance of their language does not prove they have none. Today, the study of animal communication has shown rich signal systems, but it is still debated whether they carry thoughts in Descartes's sense (our summary).
J
Jean de La Fontaine 1679 · attacks premise 3

In his poem to Madame de La Sablière, he sums up the thesis (the beast is a machine, everything happens "by springs", with no feeling or soul), then gives examples that trouble it. An old pursued stag sends a young stag in his place to mislead the hounds: "What reasoning to save his life!" For him, such cunning is worthy of the greatest generals and does not look like clockwork.

Descartes replies
Descartes 1637 · Descartes (1637 text, written before the objection)
He remarks that several animals show more skill than we do in some actions, but none at all in many others. So what they do better than we do does not prove they have a mind: otherwise they would have more than we do in everything. It is rather nature that acts in them, as in a clock.
Where the debate stands (our summary)Open point. La Fontaine thinks the stag's cunning is reasoning. Descartes replies in advance that skill in one case does not prove mind. But the reply does not say why a skilful act that adapts to a new situation (the stag facing hounds) would be better explained by springs than by thought: that is what La Fontaine disputes (our summary).
Jeremy Bentham 1789 · attacks the conclusion

That is not the right question. A full-grown horse or dog is beyond comparison a more rational, as well as a more conversable animal, than an infant of a day, or a week, or even a month old. But suppose the case were otherwise, what would it avail? The question is not, Can they reason? nor, Can they talk? but, Can they suffer?

no reply
see the argument
No reply possible: objection raised in 1789, after the author's death (1650).
Where the debate stands (our summary)Open point. Even if Descartes were right about thought, Bentham says that settles nothing: what matters morally is the capacity to suffer. But Descartes denies precisely that the beast feels pain as we do: for him, without thought there is no felt pain, only a movement of the body. Everything depends on whether a sensation can be lived without thought in Descartes's sense (our summary).
C
Cambridge Declaration on Consciousness 2012 · attacks the conclusion

A group of neuroscientists declared that convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. For Descartes, to think means everything one is conscious of: if the declaration is right, animals think in the Cartesian sense.

no reply
No reply possible: objection raised in 2012, after the author's death (1650).
The debate goes on (our summary) The text speaks of brain bases of consciousness and of affective states; it is not direct proof: it moves from a likeness of brains to a likeness of experience. Descartes could have replied that likeness of organs proves nothing (it is the only reason he sees in favour of thought in beasts, letter to More). Yet he defines thought by consciousness (Principles I, 9): the disagreement is therefore about the facts, no longer about words (our summary).
Where the debate stands (our summary)Open point. The 2012 declaration is a statement by a group of researchers, not a consensus of all science. Which animals have lived experience remains debated (our summary).
N
New York Declaration on Animal Consciousness 2024 · attacks the conclusion

Twelve years later, researchers and philosophers wrote that there is strong scientific support for attributing conscious experience to other mammals and to birds. They add that there is at least a realistic possibility of conscious experience in all vertebrates and many invertebrates.

no reply
No reply possible: objection raised in 2024, after the author's death (1650).
Where the debate stands (our summary)Open point. The 2024 declaration is careful: it speaks of strong support for mammals and birds, and only of a realistic possibility for the others. It does not claim the question is closed (our summary).