From the Cogito Descartes extracts a general epistemic rule: whatever is very clearly and distinctly… — René Descartes

From the Cogito Descartes extracts a general epistemic rule: whatever is very clearly and distinctly apprehended is true, because the certainty of his first self-knowledge depends solely on such clear and distinct perception.

By René Descartes, from Meditations on First Philosophy

Key Arguments

He reflects that in his first certain knowledge (‘I am, I exist’) nothing grounds its truth except his clear and distinct perception of what he affirms, indicating that clarity and distinctness are the sole basis of its certainty.
He notes that clear and distinct perception would not suffice for truth if it could ever happen that something thus perceived proved false, so the Cogito implicitly certifies the reliability of such perception.
On this basis he formulates a rule that he may now take generally, namely, that all that is very clearly and distinctly apprehended is true.
He distinguishes this from earlier beliefs he took as clear (about earth, sky, stars, etc.), explaining that what he truly perceived in them was only that ideas of these objects were in his mind, not the existence or resemblance of external things, so the earlier ‘clarity’ was illusory and did not meet the standard.

Source Quotes

Now, as I am endeavoring to extend my knowledge more widely, I will use circumspection, and consider with care whether I can still discover in myself anything further which I have not yet hitherto observed. I am certain that I am a thinking thing; but do I not therefore likewise know what is required to render me certain of a truth? In this first knowledge, doubtless, there is nothing that gives me assurance of its truth except the clear and distinct perception of what I affirm, which would not indeed be sufficient to give me the assurance that what I say is true, if it could ever happen that anything I thus clearly and distinctly perceived should prove false; and accordingly it seems to me that I may now take as a general rule, that all that is very clearly and distinctly apprehended (conceived) is true.

I am certain that I am a thinking thing; but do I not therefore likewise know what is required to render me certain of a truth? In this first knowledge, doubtless, there is nothing that gives me assurance of its truth except the clear and distinct perception of what I affirm, which would not indeed be sufficient to give me the assurance that what I say is true, if it could ever happen that anything I thus clearly and distinctly perceived should prove false; and accordingly it seems to me that I may now take as a general rule, that all that is very clearly and distinctly apprehended (conceived) is true. Nevertheless I before received and admitted many things as wholly certain and manifest, which yet I afterward found to be doubtful.

And even now I do not deny that these ideas are found in my mind. But there was yet another thing which I affirmed, and which, from having been accustomed to believe it, I thought I clearly perceived, although, in truth, I did not perceive it at all; I mean the existence of objects external to me, from which those ideas proceeded, and to which they had a perfect resemblance; and it was here I was mistaken, or if I judged correctly, this assuredly was not to be traced to any knowledge I possessed (the force of my perception, Lat.). But when I considered any matter in arithmetic and geometry, that was very simple and easy, as, for example, that two and three added together make five, and things of this sort, did I not view them with at least sufficient clearness to warrant me in affirming their truth?

Key Concepts

I am certain that I am a thinking thing; but do I not therefore likewise know what is required to render me certain of a truth?
In this first knowledge, doubtless, there is nothing that gives me assurance of its truth except the clear and distinct perception of what I affirm
if it could ever happen that anything I thus clearly and distinctly perceived should prove false
and accordingly it seems to me that I may now take as a general rule, that all that is very clearly and distinctly apprehended (conceived) is true.
But there was yet another thing which I affirmed, and which, from having been accustomed to believe it, I thought I clearly perceived, although, in truth, I did not perceive it at all; I mean the existence of objects external to me

Context

Early in Meditation III, immediately after recalling the Cogito, Descartes analyzes what made that knowledge indubitable and generalizes it into the rule of clear and distinct perception.