Alvin Plantinga’s notion of warrant (justification) is a form of externalism (reliablism).
[A] belief has warrant only if it is produced by cognitive faculties that are functioning properly in an appropriate environment. Plantinga’s notion of proper function, moreover, implies the existence of a design plan, and a belief’s having warrant requires that the segment of the design plan governing the production of the belief is aimed at truth. In addition, the design plan must be a good one in the sense that the objective probability of the belief’s being true (given that it’s produced in accordance with the design plan) must be high.
However, Plantinga does not avoid the Gettier problem. The Gettier problem challenges the notion that knowledge is a justified true belief. In short, the problem is about accidental knowledge ([K], if there is such a thing) and having a belief that is true while your reason for justification is false.
Scenario: Mary has good eyesight, but it is not perfect. It is good enough to allow her to identify her husband sitting in his usual chair in the living room from a distance of fifteen feet in somewhat dim light. This situation has happened before many times with properly functioning properties.
The goal belief for Mary: “My husband is sitting in the living room.” (B1) Usually, Mary has enough warrant for this belief to be true. Suppose Mary misidentifies the chair-sitter who happens to be her husband’s brother. The environment is not abnormal. There is no intention of trickery. Mary’s faculties are functioning, as they should be, though not necessarily perfectly. This is contra-fake barns since that scenario is abnormal (Plantinga’s example). She may have good reason to believe that her husband’s brother is not present, even though she knows they look similar, and that he may actually be in Australia.
The Gettier problem plagues Plantinga’s warrant. Mary’s husband could be sitting on the other side of the room, unseen by her. In that case her belief B1 is true and has sufficient warrant for K on Plantinga’s account, but she does not have K. Plantinga on the problem: “What is essential to Gettier situations is the production of a true belief despite a relatively minor failure of the cognitive situation to match its design.”
Warrant functions to a degree, but the degree of warrant sufficient for K does not require faculties to be working perfectly in an environment perfectly matched to them.
Either the degree of warrant is sufficient for K, or it is not. If it is not, then a multitude of beliefs we normally think we are warranted are not and there is much less K in the world than Plantinga’s numerous examples suggest. On the other hand, if the degree of warrant is sufficient for K, then Plantinga’s theory faces Gettier problems structurally identical to those of other theories.
Even if some aspect of the Mary scenario makes it unpersuasive, there must still be cases of warranted false belief if the component of truth in K is not redundant. With such a case, Gettier examples can be present by adding a feature extraneous to the warrant of the belief, which makes the belief true after all. Simply add happenstantial luck. Plantinga should have responded to the Gettier problem in that the problem is due to a relatively minor failure of the cognitive situation to connect to the truth.
For more on this discussion and the above material please read Linda Zagezebski’s paper ” The Inescapability of Gettier Problems.”