Reliabilism holds that a belief is justified if and only if it is formed using a reliable method. Because reliabilism is an externalist theory of justification, i.e. a theory that says that whether or not a belief is justified depends on factors external to the understanding of the believer, it is open to criticism on the ground that it is unfair.

When we are deciding what to believe, we can only be expected to take into account evidence to which we have access. This includes our own experiences and background beliefs, for example, but does not include the reliability or otherwise of the various belief-forming methods available to us.

Whether or not a given method of forming beliefs is reliable is not necessarily something that we know. It may be that although all of our evidence suggests that perception is reliable, it isn’t, and in such circumstances, it would be unfair to say that we aren’t justified in our beliefs based on perception. Similarly, it may be that although all of our evidence suggests that astrology is unreliable, it’s actually an excellent guide to truth, and in such circumstances, again, it would be a mistake to say that beliefs based on astrology are justified.

Justification involves doing our epistemic duty, forming beliefs in the way that we ought to form them. Reliabilism suggests that justification depends on factors beyond our understanding, and so asks far too much of us. We can’t be expected to take such factors into account when deciding what to believe. Epistemic justification must therefore depend on factors internal to the understanding of the believer, and these do not include the reliability or otherwise of belief-forming methods.