How can we know there isn't an illusion of thought?

This question stems from Descartes' assertion of "I think, therefor I am." He regards the notion that he thinks as an absolute fact. But how does he know that? How does he know that thought isn't just an illusion?

Thanks in advance!


2 Answers 2


There are many critiques of Descartes' famous phrase, see here:


Briefly, some attack the presupposition of an "I" (Georg Lichtenberg). Others criticize him placing "thinking" logically prior to Existence (Søren Kierkegaard). And Friedrich Nietzsche denied that he could assert anything at all but that "something thinks".

Here the doubt is placed on the "I"'s ability to trust his perception of his thoughts. Descartes actually contemplating this when asking, how can I be sure of the reality of my perceptions; and then concluding that he must doubt everything, which led to the discovery the 'he' couldn't doubt that 'he' was doubting. Note the arguably more correct form his conclusion:

I doubt, therefore I am

Of note also is that Descartes did not originate the idea, as far back as Plato, some form of this have been cropping up ever since. Note for instance:

Augustine of Hippo in De Civitate Dei writes Si […] fallor, sum ("If I am mistaken, I am") (book XI, 26)


Consider what you are saying here. You are willing to accept that there can be such a thing as an "illusion" as a premise for your question. However, you're not convinced that there can be thought.

An "illusion" is an instance of a misapprehended perception of some sensory input, so in order to use this concept of an illusion, you have already accepted (as a premise to your question) that we have senses, we have perceptions of sensory inputs, we have apprehensions of those sensory inputs, and there is a possibility of mis-apprehension of those sensory inputs. In short, you have already accepted that we have sensory inputs and then we think about them, and our conclusions about these sensory inputs can be compared to some objective external reality to determine if they are correct apprehensions, or merely "illusions". When you use the concept of an "illusion" in your argument you have already accepted all of these preliminary positions; even by using this concept of "illusion" you have already accepted that we're thinking. (Or to put it another way, if there were no thought, there could not be a valid concept of "illusion".)

This kind of attempt to deny the existence of thinking is an example of the "stolen concept fallacy". It is not possible to deny the existence of consciousness and thought, without invoking concepts that logically depend on the existence of these things. Any argument against thought must involve the use of concepts that pre-suppose thinking. For this reason, the existence of thought is an axiom --- a concept that is necessarily taken as a premise to all other observations and arguments.

  • Surely most thinking could be fine, but some of it illusion?
    – CriglCragl
    Commented Jul 16, 2018 at 9:11
  • Of course it is possible for some thinking to be illusion; but it is not possible for there to be such a thing as an illusion if there is no such thing as thinking.
    – Ben
    Commented Jul 16, 2018 at 12:55

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