Full Idea
A modified version of reliabilism proposes that a belief is justified in case it is the product of a process that is working according to its 'proper function' in the environment for which it is appropriate.
Gist of Idea
Maybe a reliable justification must come from a process working with its 'proper function'
Source
report of Alvin Plantinga (Warrant and Proper Function [1993]) by J Pollock / J Cruz - Contemporary theories of Knowledge (2nd) §1.5.4
Book Reference
Pollock,J.L./Cruz,J: 'Contemporary Theories of Knowledge (2nd)' [Rowman and Littlefield 1999], p.26
A Reaction
Something might infallibly indicate something without that being its proper function (e.g. 'Red sky at night/ Shepherds' delight'). An inaccurate clock is fulfilling its proper function (telling the time), but not very well.