Doctrine that knowledge must have foundations; that is, if we are to know anything at all there must be some things that we can know incorrigibly, so that it is impossible - or perhaps does not even make sense - for us to be mistaken.
The usual candidates for such knowledge have been facts about the immediate data of the senses or of introspection, such as what colors, tastes, and so on, are currently present to us, or what state of feeling or state of mind we are in.
But probably the most famous foundationalist statement has been that of Rene Descartes (1596-1650): 'Cogito ergo sum' ('I think therefore I am'). One objection to foundationalism is that it is hard to make knowledge incorrigible without constricting it so far that it ceases to be knowledge at all (one can misclassify even one's own immediate experiences).
R Descartes, Meditations on the First Philosophy (1641), especially Meditations 1 and 2