In his paper, "Plantinga's Model of Warranted Christian Belief"[1}, Christian philosopher James K. Beilby raises an interesting and powerful criticism of Plantinga's latest and most mature version of his Reformed epistemology. First, though, some review and stage-setting:
According to Plantinga's account, a belief must satisfy four conditions if it is to have at least some measure of warrant:
(i) The belief must be produced by properly functioning cognitive faculties.
(ii) The relevant cognitive faculties must be successfully aimed at truth.
(iii) The belief must be produced in an epistemically congenial maxi-environment and mini-environment
(iv) The belief is subject to no undefeated defeaters (i.e., reasons against the belief that have yet to be undercut or rebutted).
So that's what's required for a belief to have any warrant at all. But Plantinga allows that warrant admits of degrees, and he ties the degree of warrant a belief enjoys to the degree of firmness with which it is believed. Thus, for a belief to have a degree of warrant sufficient for knowledge, it must be held with a very high degree of firmness.
Putting it all together, Plantinga's account can be summed up as follows:
I. Conditions of warrant are met + high degree of firmness = high degree of warrant.
II. Conditions of warrant are met + low degree of firmness = low degree of warrant.
So that's Plantinga's account of warranted belief in a nutshell. But how does this account connect to his account of warranted Christian belief in particular?
Very (very!) roughly, on Plantinga’s extended A/C model of warranted Christian belief, the internal instigation of the Holy Spirit acts on the believer by repairing the sensus divinitatis and producing a firm and unwavering faith in the Great Things of the Gospel. And since such belief meets all of the conditions of warrant (viz., (a) the proper function condition, (b) the truth-aimed faculty condition, (c) the epistemically congenial mini- and maxi-environments condition, and (d) the no defeaters condition), the degree of warrant enjoyed by the believer’s belief is sufficient to constitute knowledge. Enough review and setup; on to Beilby's criticism of Plantinga's model of warranted Christian belief.
In the paper mentioned above, Beilby argues that there is a tension between the extended A/C model's depiction of the paradigmatic believer's belief as firm and unwavering, on the one hand, and the actual facts about the typical believer's state of belief, on the other. For contrary to Plantinga's account, the belief of many believers is weak and wavering -- the “I do believe; help thou mine unbelief” sort. This is a potentially fatal problem for Plantinga's latest incarnation of reformed epistemology. For if we can't reconcile the model with the data, then Plantinga's model fails the minimal standards that he himself sets for it, viz., that it be at least epistemically possible (i.e., compatible with what we know or have reason to believe is true about the world.).
Unfortunately, it's hard to see how the tension between his model and the data can be resolved in principle. So, for example, it’s implausible (and blasphemous) to say that the Holy Spirit fails in his job to produce sufficiently warranted Christian belief in many believers; nor is it plausible to say that the multitude of believers with less-than-fully-firm faith are actually non-believers. What, then, can explain the data of variability in degree of belief among Christians? Beilby points out that Plantinga chalks up the less-than-maximal belief in the typical believer to the noetic effects of sin. Will this reply solve the tension?
As Beilby points out, it will not. For this would mean that a certain portion of the relevant cognitive faculty's mini-environment (their sin-racked body and mind)) isn't epistemically congenial, in which case the belief fails the “congenial epistemic environment” condition of his account of warrant. As such, those with less than maximal faith have a warrant-defeater for their Christian faith.
 in Peter-Baker, Deane. Alvin Plantinga (Cambridge University Press, 2007), pp. 125-165.
 "Put in a nutshell, then, a belief has warrant for a person S only if that belief is produced by cognitive faculties functioning properly (subject to no dysfunction) in a cognitive environment that is appropriate for S's kind of cognitive faculties, according to a design plan that is successfully aimed at truth." (Warranted Christian Belief, p. 156)
 "We must add, furthermore, that when a belief meets these conditions and does enjoy warrant, the degree of warrant it enjoys depends on the strength of the belief, the firmness with which S holds it." (Ibid.).
As Beilby notes on a related matter: "While it is undoubtedly easier to describe and defend the warrant of "epistemological saints", because the Extended A/C model describes the ideal, fully formed faith of paradigmatic believers rather than the usual, in-process faith of typical believers, Plantinga's attempt to use the Extended A/C model to provide a good way for Christians (including, I assume, typical Christians) to think about the epistemology of Christian belief is in jeopardy. Since the faith of typical believers looks very different from that described in Plantinga's model, they have a choice between questioning the warrant of their belief about God or rejecting Plantinga's model as a good explanation of the warrant of their religious beliefs. Since Plantinga himself argues that the beliefs of "most Christians" are "both externally rational and warranted", the most reasonable option for the typical Christian is the latter." Ibid., p. 146.
 Alternatively, the problem could be chalked up to malfunction in the relevant cognitive faculties, due, again, to the ravishes of sin. But then a different condition of Plantinga's account of warrant isn't met -- viz., the proper function condition -- in which case, again, the belief isn't warranted, according to the conditions of warrant laid down by Plantinga himself.
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