Jamie asked:

Descartes in his meditations tells us that an evil demon controls all our perceptions, but the meditator retains the power willingly to suspend judgment, the cartesian demon cannot simply force its victim to have any arbitrary chosen belief. For example, it cannot install and run an arbitrarily chosen train of thought in the mind of the philosopher thereby making the philosopher believe whatever the demon wants. But how could Descartes be sure of that? If this was the case in our reality how would we know or discover it? and how could we deal with knowing it? I guess what I’m asking is are our thoughts and perceptions and even actions controlled by us?

Answer by Jürgen Lawrenz

Descartes can be sure because at the end of the demon’s insinuations there is no Descartes left, only a ‘thinking thing’. This res cogitans knows nothing other than that ‘it is’. As a self-contradiction results from its assertion of self-nonexistence, Descartes has now the option of assuming that this thinking thing, which has the capacity of knowing its own existence indubitably, must be an individuated thinking thing, i.e. a soul. From this it follows logically that the soul has at least one clear and distinct perception, namely of itself as an existent; and as this one thought does not exhaust the soul’s capacity for thinking other thoughts, it invites the conclusion that all equally clear and distinct perceptions of the soul would be equally indubitable, irrespective of any demon’s insinuations.

Candidates for such indubitability might be mathematical truths or mechanical laws and metaphysical ideas such as, ‘I cannot be the sole existent in the world’. As these evolve from the inside, i.e. from the soul itself, they do not depend on external verification, and therefore the demon has no power to demolish them. Accordingly there is a species of thought (ideas) that has power to validate perceptions by the senses. These include phenomena with features which are delivered to the mind and exhibit geometrical ratios, measurable volumes etc. Accordingly the back door is now open to the verification of sensory perceptions through the application of such truths and laws. Similarly, the mind has power to dispel superstitions, hallucinations and the like, because e.g. witches perform tasks which are impossible under mechanical laws and ghosts are phenomena without geometrical features.


Answer by Geoffrey Klempner

At one point in the First Meditation, Descartes considers the possibility that he is mad — and dismisses the thought without further argument. Was he right to do so? If an evil demon is messing with my very thought processes then the game is up. There is nothing I can do to defend my mind against the power of the evil demon.

Arguably, the over-arching assumption of the Meditations, which is never challenged, is that this is an exercise in reasoning. Whatever illusions and misinformation I may be bombarded with (by an evil demon, e.g.) the response comes from the exercise of reason.

So you are right that scepticism could be taken further than Descartes takes it. But the point was never simply to battle with the full-on determined sceptic; it was to use the idea of methodological scepticism as means of discovery of foundational metaphysical truths.

I think that the notion of an evil demon is incredibly important in metaphysics (still!) and have made a YouTube video on this topic, Return of the evil demon which you might be interested to watch.