Rene Descartes
Before Descartes, philosophy of perception was much more straightforward - you either saw something or you didn't.
Descartes noticed how hard it is to differentiate between seeing something in a dream and seeing the same thing in waking life (could give eg here). Our senses can obviously be deceived, so how do we know we're not being deceived all the time? How can we tell when we're dreaming and when we're awake?
He reached his infamous conclusion, arguing that even the most convinced sceptic must be certain of one's own existence as a thinking thing:
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