Descartes seemed to have the following problem:

At this point I had dwelt on this issue to show that if there were such machines having the organs and outward shape of a monkey or any other irrational animal, we would have no means of knowing that they were not of exactly the same nature as these animals, whereas, if any such machines resembled us in body and imitated our actions insofar as this was practically possible, we should still have two very certain means of recognizing that they were not, for all that, real human beings.

He then gives a language based argument which should establish a difference between Humans and machines. He gives a quick rundown of the argument, which is as follows:

The first is that they would never be able to use words or other signs by composing them as we do to declare our thoughts to others. ... it is not conceivable that it should put these words in different orders to correspond to the meaning of things said in its presence, as even the most dullwitted of men can do.

So it seems he wants to argue that language, specifically expressing thoughts, is a distinguishing factor for humans when compared to machines. He further claims that it distinguishes humans from animals too, and provides an argument for this idea.

In the argument, he specifically claims that

[The fact that animals cannot let their thoughts be known and humans can] shows not only that animals have less reason than man, but that they have none at all.

He then adds,

For it is clear that we need very little reason in order to be able to speak; and given that as much inequality is found among animals of the same species as among men, and that some are easier to train than others, it is unbelievable that the most perfect monkey or parrot of their species should not be able to speak as well as the most stupid child, or at least a child with a disturbed brain, unless their soul were of a wholly different nature to ours.

For starters, to me it seems the above is support for the conclusion that they have no reason, since it begins with "For it...". But I cannot make sense of the rest...

The reason for the first part is clear; it is necessary that speech requires little reason for his argument to make sense. But what is the point of the rest of the statement? How does it help establish his conclusion? To me it seems like an implication, that if animals had the same soul as us then they would be capable of expressing their thoughts. And since he has already established that they cannot do that, they must have a different soul. But how does this help support the notion that they have no reason? Is the conclusion NOT that animals have no reason? If so, what was the point of stating that animals have no reason in such a conclusion-like manner?

With that being said, there is still one more odd point he makes. He mentions that:

given that as much inequality is found among animals of the same species as among men, and that some are easier to train than others

Why is this included? I do not get it at all. If this was removed, the argument does not seem to lose anything to me...

Overall, I do not understand this last part of the argument (and really the argument as a whole). It seemed up until this point that he was arguing that animals lack reasoning, but this last part of the argument seems to suggest something else. Or it somehow is suppose to support that animals have no reasoning, but I have no idea how.

2 Answers 2


The point is basically that the difference is not one of grade but one of kind:

When he points out that there is a lot of difference (a wide spectrum) in ability within a given species, this strengthens his argument insofar as it supposedly makes clear that even the cleverest of animals has not as much reason as the most "dullwitted" human. This speaks for a categorical difference, which he calls the possession of reason.

Thus, his point here is that since we observe a wide variety within species and yet no amount of variety allows animals to possess language, therefore Reason as a categorical difference between humans and animals makes for a better explanation.

Mind, back in his days any condition so severe that it precludes learning language in a social environment would have lead to death during the pregnancy or shortly thereafter.

  • This makes a lot of sense, thank you! One thing I am still curious about is his mention of the animal soul being different in nature (or not depending on translation) compared to ours. Is this just explanatory fluff, or does it serve some other purpose in this argument? I.e. why did he include that little soul part at the end of the broader argument? Is it meant to explain, perhaps, why animals do not have speech?
    – Curulian
    Sep 19, 2023 at 6:13
  • 1
    @Curulian The metaphysical background is that Descartes (in Scholastic tradition) sees reason as the soul being able to see the light of God, ie. it is a categorical difference between the souls that enables man to have reason, as being able to partake in divine insight.
    – Philip Klöcking
    Sep 19, 2023 at 6:16

The idea is there are very clever animals (monkey, dog, or even horse) but none of those can talk or write their thoughts, but the most stupid human can do that. So, in Descartes' mind, it must not be a difference of degree in inteligence, but indeed a difference in the quality of our souls (or the lack thereof in the case of animals).

This argument is riddled with problems, some of them Descartes can't be blamed for as at his time he didn't have access to the research about neurology and cognition we now consider granted, others are frankly difficult to forgive even to a XVII century man.

The unforgivable one being that he completely glosses over the fact there are humans who can't speak nor write because of some form of handicap. What does he make of those? Do they have no soul? This is what his argument points to but he conveniently avoid the question (Maybe I am being too harsh and the survival rate of so heavily handicapped children was too low in his time for him to have studied their case, anyway the objection to the argument remains valid).

The forgiveable ones are that recent progress in AI have shown that computers, although they are soulless as far as we know, can mimick human language to the point that laypeople are mislead into thinking they actually are using reason like us. The fact that we can witness actual machines "use words or other signs by composing them as we do to declare our thoughts to others" completely breaks the argument's premise.

The other is that what makes animals unable to speak has little to do with their purported inability to reason, and more to do with the fact they lack the cognitive and physiological capacity to understand and formulate articulate speach, both of which we have now identified and know to have evolved in parallel in prehominids and early homo sapiens.

  • So, the whole point of the argument was to conclude that animals have a soul with a different nature to ours (ours being the "rational soul," whatever that is)? So why did he go through all the trouble of concluding that animals have no reason at all? Did he just forget to mention that not being able to reason = not having a human soul, and also that being able to reason = the ability to express thought (speak)? When reading his argument, it felt like the punchline was that animals have no reason, but then he gave a side point that also animals have a different soul. Is that wrong?
    – Curulian
    Sep 19, 2023 at 3:12
  • @Curulian My understanding is he argues animals have no reason because they never express a reasonned argument (which, for some reason, he holds to be different than the ability to invent solutions to new problems, which we know animals have). And since they have no reason it must be because we have a soul they don't have. But as i said it's incredibly weak even by Descartes' times standard, because there are humans who don't have the ability to reason, and we know now that what animals lack is not some immaterial soul but identifiable parts in the brain (and other physiological traits).
    – armand
    Sep 19, 2023 at 3:28

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