Buchak defines faith as “being willing to act on a claim without further evidence and to remain committed to acting on that claim even when counterevidence arises.” So, to have faith that a friend is trustworthy is to be willing to trust them even though you do not have any specific evidence that they are trustworthy. You may have some general evidence, such as the fact that they have never betrayed your trust in the past, but you are willing to act on the claim that they are trustworthy even if you do not have any specific evidence to support that claim.

Buchak argues that faith can be rational in certain circumstances. For example, if you have a lot of past evidence that a friend is trustworthy, then it may be rational to have faith in them even if you do not have any specific evidence to support that claim in the present. This is because it is likely that they will continue to be trustworthy in the future, given their past behavior.

However, faith is not always rational. If you have no evidence at all that a friend is trustworthy, then it is not rational to have faith in them. This is because it is possible that they are not trustworthy, and you could end up being hurt if you trust them.

Ultimately, whether or not it is rational to have faith in a friend is a judgment call that each individual must make for themselves. There is no easy answer, and the decision will depend on the specific circumstances of the situation.

Here are some additional points that Buchak makes about faith:

• Faith is not the same as blind faith. Blind faith is belief without evidence. Faith, on the other hand, is belief based on evidence, even if the evidence is not conclusive.
• Faith can be rational even if it is not based on certainty. We often make decisions based on probabilities, and faith is no different. We may have a high degree of confidence that a friend is trustworthy, even if we cannot be certain of it.

Faith can be costly. If we trust someone who turns out to be untrustworthy, we can get hurt. However, the potential benefits of faith can outweigh the costs. For example, if we trust our friends, we can build strong relationships with them.