Recently physician overconfidence has been considered as a major factor contributing to diagnostic error. A philosophical inquiry into overconfidence as a character vice has the promise of shedding light on how we can overcome this vice and potentially reduce diagnostic errors. In his recent work, Quassim Cassam conducts such an inquiry. This paper puts Cassam’s work on physician overconfidence in the context of his theoretical work on self-knowledge and epistemic vices. It shows that physician overconfidence, considered as a major factor in diagnostic error, provides a significant real-life application of Cassam’s accounts of self-knowledge and epistemic vices. It focuses on the features of these accounts that lead to the following result: self-knowledge is rarely, if ever, a remedy for physicians’ overconfidence and the resulting diagnostic errors. By appealing to the same data Cassam cites regarding diagnostic error and physicians’ overconfidence, it is shown that the more substantial—in the sense Cassam specifies—one’s third-personal knowledge of oneself is, the less likely it is to be of any practical value qua self-knowledge. This paper defends the view that what Cassam calls ‘trivial self-knowledge’—first-personal knowledge that has been the primary concern for philosophers—is crucial for any kind of self-knowledge to be instrumental for self-improvement. Since an agent acts from the standpoint that she is aware of herself trivially, it is argued that what Cassam calls ‘substantial self-knowledge’ has no practical value unless it is integrated with what he calls ‘trivial self-knowledge’. In this way the paper explains why if what one learns about oneself from the third-person perspective is drastically different from what one takes to be true from the first-person perspective, one cannot act on this knowledge. Since the standpoint from which one experiences and acts intentionally are one and the same, the paper also explains why traumatic experiences (such as the death of a patient or of a loved one) can sometimes lead to fundamental change and self-improvement.