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In the context of epistemology, the reliability of a method of gaining knowledge refers to the extent to which the method can be depended on to produce true results.
Science is often put forth as the most reliable way of developing an understanding of the world around us because its conclusions are based upon, and continually checked against, observed facts.
Esoteric ways of knowing such as divine revelation are, on the other hand, quite unreliable, as can be seen by comparing the different conclusions reached by different people or groups purporting to receive revelation from the same divine source. (See, for example, the many denominations of Christianity.)