René Descartes (1596-1650). The popular version of Descartes. Meditations on First Philosophy (1641). One of only four works published during Descartes’ lifetime (the others are Discourse , Principles , Passions of the Soul 
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“inasmuch as reason already persuades me that I ought not less carefully to withhold my assent from matters which are not entirely certain and indubitable than from those which appear to me evidently to be false, if I am able to find in each one some reason to doubt, this will suffice to justify my rejecting the whole.”
“for that end it will not be requisite that I should examine each in particular, which would be an endless undertaking; for owing to the fact that the destruction of the foundations of necessity brings with it the downfall of the rest of the edifice, I will only in the first place attack those principles upon which all my former opinions rested.”
Descartes uses the method of what is sometimes called hyperbolic doubt (i.e. exaggerated doubt for a specific purpose); he does not say “doubt everything” or “treat every former opinion as false”, but rather “doubt everything or treat everything as false until proven otherwise”.
In addition to optical illusion, Descartes appeals to various other instances where we can be mislead by sensory information – obscure conditions, hallucination, phantom pains – all of which it is possible dismiss as cases of non-optimal or non-standard conditions of perception
Q: if ‘optimal’ conditions are those where we don’t go wrong, how can we sure we’re in the optimal situation?
However much we might come to doubt the reliability of sense information (and, at least provisionally, treat all such information as false), there is a great deal we might still claim to know even under non-optimal conditions.
2. The dream hypothesis: “there are no certain indications by which we may clearly distinguish wakefulness from sleep” But if I don’t know that I am not now dreaming, how do I know that there is an external world at all?!
Again, even if we accept that we might presently be dreaming, still there are ‘truths’ (knowledge items, if you will) that escape even this fine a net (e.g. 2 +2=4)
3. The ‘evil genius’ hypothesis: What if, instead of God, there is an anti-God, who can cause me to be certain even where what I most certain about is false? What then? Is nothing certain then? Happily, there is a solution.
The challenge of the evil genius hypothesis is that we might be certain about something which is false. But there is at least one thing about which we can be certain, even if we doubt it, and that is that we exist (for we must exist, if we are doubting)
This is Descartes’ ‘Archimedean’ point; it shows that at least one thing escapes the skeptical net.
Q: Is this enough? Is it sufficient to know this one (fairly obvious) thing?