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In our quest for knowledge, Descartes suggests that we rely on pure intellectual ideas. By taking stock of these ideas and then deducing their logical results, we can arrive at all possible human knowledge. However, we must be careful not to start the process of reasoning from any old ideas we find in the intellect. Judgments can only be certain when the ideas concerned are clear and distinct. Only when a perception is clear and distinct can we proceed from that perception to knowledge. Clear and distinct perceptions, therefore, are the single most important tool in Descartes’s toolkit.
A clear and distinct perception is just a perception to which one cannot fail to assent. So long as you are entertaining the notion, you cannot doubt it without landing yourself in a logical incoherence. So, for example, the perception that two plus two equals four would count as a clear and distinct perception. So long as you are aware of the meaning of the terms involved, you cannot coherently doubt the truth of this claim. Descartes believes that the same phenomena hold true of propositions such as “nothing can exist and not exist at the same time,” and the ever-popular “I think, therefore I am.”
Although you cannot doubt clear and distinct perceptions if they are before your mind, once they fall out of your awareness, doubt can creep back in. If you have stopped entertaining the proposition that two plus two equals four, but only remember the conclusion you reached, then you might begin to doubt the legitimacy of the conclusion. You might wonder whether your reasoning was as airtight as you had thought while it was taking place, perhaps whether some evil scientist was responsible for putting that thought in your mind. Obviously, as long as doubt can keep creeping back in whenever a clear and distinct perception falls out of awareness, clear and distinct perceptions are not going to be much help in the search for knowledge.
Descartes, therefore, appeals to God to guarantee the truth of clear and distinct perceptions, so that we can believe in them even after we have stopped entertaining them. God, he claims, created us and, thus, also our faculty of reason. He is, therefore, responsible for our clear and distinct perceptions. In addition to being our creator, God is also infinitely perfect. If our clear and distinct perceptions were not trustworthy, though, God would be far from perfect. He would be a deceiver, mean and malicious. An infinitely perfect God would never give us a faculty that presented perceptions as indubitably true when really, they were false. Therefore, we can trust our clear and distinct perceptions. So long as we remember that a conclusion was reached through a perception that was clear and distinct (i.e., indubitable) while it was going on, we can be absolutely certain that the conclusion is true.