René Descartes

"I am certain that I am a thinking thing. But do I not therefore also know what is required for me to be certain of anything? Surely in this first instance of knowledge, there is nothing but a certain clear and distinct perception of what I affirm. Yet this would hardly be enough to render me certain of the truth of a thing, if it could ever happen that something that I perceived so clearly and distinctly were false. And thus I now seem able to posit as a general rule that everything I very clearly and distinctly perceive is true."​\

• Above​ is a passage from Meditations of the First Philosophy

• This passage is a clear example of his thoughts on his method of doubt

  • Describes how what may be seen as true can be false​

  • Only what he can obviously understand is true

  • How descartes advances his works