We can ask why self-justifying reasons are self-justifying. If the traditional foundationalist has an answer, it seems like it must involve some metajustificatory feature. If the traditional foundationalist has no answer, it seems like the view has arbitrary foundations. (See BonJour, Structure, 30-3, for a similar argument.)
However, the traditional foundationalist can argue that completely self-justifying reasons are not self-justifying in virtue of some metajustificatory feature, nor are they arbitrary. It may be that certain reasons have to be assumed to be self-justifying if skepticism is to be avoided. This is a rather familiar form of rationalist argument for the existence of a priori justification. Here, the main implication of these arguments is that there might be a way to non arbitrarily show that we need to take certain reasons to be completely self-justifying without requiring that there be a metajustificatory feature which makes those reasons self-justifying. What convinces us we need to take those reasons to be self-justifying need not make them self-justifying.
This move does not seem to be available in the case of reasons that are self-justifying only to a degree. (pg. 544)