Critical thinking is reasonable and reflective thinking aimed at deciding what to believe or what to do. In Chapter 1, we saw that thinking critically requires having good reasons for the decisions we make and the beliefs we form, and we saw that reasons are good if they are both acceptable and sufficient. Since critical thinking is aimed at knowledge, the kinds of reasons it requires are epistemic reasons, which are more commonly called “evidence.” In Chapter 2, we learned how to think critically about the meaning of claims, plans, and proposals. In Chapter 3, we studied what it is for our evidence to be sufficient and we saw that a valid argument provides the ideal amount of logical support. In Chapter 4, we saw that our evidence is acceptable when it comes from a reliable source that we have no good reason to doubt, or when it is itself sufficiently supported by acceptable evidence. If we rely only on sufficient and acceptable evidence for beliefs and plans that are as clear as we can make them, then we have pretty much done our duty as critical thinkers. Though our beliefs might still be mistaken and though our plans may yet fail, we will have done the most we can in advance to avoid this. We will have pretty much fulfilled our intellectual obligations as critical thinkers.

But it would be nice to have easier ways to tell when our reasons are sufficient. In Chapter 3, we studied the notion of validity, ...

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