Reliabilism, a category of theories in the philosophical discipline of epistemology, has been advanced as a theory of knowledge, both of justification and of knowledge. Process reliabilism has been used as an argument against philosophical skepticism, such as the brain in a vat thought experiment. Process reliabilism is a form of epistemic externalism. A broadly reliabilist theory of knowledge is roughly as follows:

One knows that p (p stands for any proposition--e.g., that the sky is blue) if and only if p is true, one believes that p is true, and one has arrived at the belief that p through some reliable process.