A Penny and a Diamond
There is an important difference between psychological certainty and epistemic certainty. The former is a property of persons. One attributes this property to a person when one says: “She is certain that the team will win the game tomorrow” and “He claims to be certain that he put gas in the tank last week” and “Jones says he’s certain that he had an experience of the supernatural yesterday.” This sense of certainty is more precisely referred to as ‘complete confidence’ or ‘being absolutely convinced’ or ‘being totally sure.’ This is a subjective type of certainty. It is often not justified, since much of what folks claim psychological certainty about might turn out false. Hominum sententia fallax. The team might lose the game. The supposedly supernatural experience might not be about the supernatural after all.
The latter is about propositions. We say: “That 2+2=4 is certain” and “It is certain that red is a color” and “The law of non-contradiction is a matter of certainty.” This sense of certainty concerns a proposition’s being guaranteed to be true for a person given that person’s evidence for it. In other words, p is epistemically certain for S iff S cannot be wrong that p given S’s evidence for p. This is an objective type of certainty. It’s not about the mind of the believer, but instead about the proposition believed.
Psychological certainty is common but inadequate, like an old penny. It’s easy to find, easy to lose, and won’t buy much. It does not guarantee the possession of truth. Given human fallibility, it is unreliable. Epistemic certainty is rare but highly valuable, like a diamond. It does guarantee truth. It is completely reliable.