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"The problem of the criterion" seems to me to be one of the most important and one of the most difficult of all the problems of philosophy. I am tempted to say that one has not begun to philosophize until one has faced this problem and has recognized how unappealing, in the end, each of the possible solutions is. I have chosen this problem as my topic for the Aquinas Lecture because what first set me to thinking about it (and I remain obsessed by it) were two treatises of twentieth century scholastic philosophy. I refer first to P. Coffey's two-volume work, Epistemology or the Theory of Knowledge, published in 1917.¹ This led me in turn to the treatises of Coffey's great teacher, Cardinal D. J. Mercier: Critériologie générale our théorie générale de la certitude.² ¶ Mercier and, following him, Coffey set the problem correctly, I think, and have seen what is necessary for its solution. But I shall not discuss their views in detail. I shall formulate the problem; then note what, according to Mercier, is necessary if we are to solve the problem; then sketch my own solution; and, finally, note the limitations of my approach to the problem.

In the recently published Life by Leslie Stephen of his brother, Fitz-James, there is an account of a school to which the latter went when he was a boy. The teacher, a certain Mr. Guest, used to converse with his pupils in this wise: " Gurney, what is the difference between justification and sanctification? — Stephen, prove the omnipotence of God!" etc. In the midst of our Harvard freethinking and indifference we are prone to imagine that here at your good old orthodox College conversation continues to be somewhat upon this order; and to show you that we at Harvard have not lost all interest in these vital subjects, I have brought with me tonight something like a sermon on justification by faith to read to you, — I mean an essay in justification of faith, a defence of our right to adopt a believing attitude in religious matters, in spite of the fact that our merely logical intellect may not have been coerced. 'The Will to Believe,' accordingly, is the title of my paper.

According to Alvin Plantinga, it has been widely held since the Enlightenment that if theistic beliefs are to be considered rational, they must be based on propositional evidence. It is not enough for the theist just to refute objections. The theist "must also have something like an argument for [such a] belief, or some positive reason to think that the belief is true." But this is incorrect, Plantinga argues. Basic beliefs are beliefs not based on propositional evidence; such beliefs are "properly basic in a set of circumstances" if they can be so affirmed in those circumstances "without either violating an epistemic duty or displaying some kind of noetic defect." And, according to Plantinga, theistic beliefs can be properly basic. For example, he argues that "under widely realized conditions it is perfectly rational, reasonable, intellectually respectable and acceptable to believe there is such a person as God without believing it on the basis of evidence — propositional evidence vs. the kind instanced by 'the evidence of the senses'." But can a properly basic belief such as this have any epistemic credibility (warrant) if it is not conferred by other propositions whose epistemic status is not in question? Yes, Plantinga replies. There are two significantly different ways in which a proposition can acquire warrant. There is propositional warrant — warrant conferred by an evidential line of reasoning from other beliefs. However, there is also nonpropositional warrant.