Mistakes occur because doctors make diagnoses and therapeutic decisions on the basis of what they think is correct knowledge but is, in fact, incorrect knowledge. Attainment of knowledge relies on sense experience, and sense experience can be misleading. Medical tests are interfaces between patients and doctors, and sense experiences are distorted by these interfaces. Knowledge can be attained by “reasoning.” Deductive reasoning, utilizing a properly constructed syllogism, is the only way one attains knowledge with 100% certainty. Knowledge attained by all other methods is only correct with a probability <100%. In addition to reasoning, one can gain knowledge on the basis of appeals to authority (eg, expert witness), experience, intuition, revelation, and faith. The level of certainty of knowledge that one can achieve is related to the method used to achieve that knowledge; deductive reasoning is more reliable, for example, than intuition, revelation, or faith. Doctors and lawyers must determine what is true or correct about matters that are uncertain or contingent. Doctors, like lawyers, deal with evidence (medical history and test results). To avoid errors, doctors must treat evidence with the same level of skepticism that lawyers do. Doctors must be aware of what constitutes quality of evidence. If they are not, medical errors will result. In summary, doctors must understand the basic principles of epistemology. Doctors never deal with 100% certain knowledge and hence must rely upon “wisdom,” which is the ability to make the right decision in the absence of certain knowledge.