It is often supposed that knowledge must be constructed upon a foundation. In this view, rational investigation consists of wiping the slate clean to begin anew, and then seeking basic beliefs which can serve as a foundation, relative to which all knowledge can be justified. The metaphor is of a building with foundations and a framework, which must be constructed piecemeal, but no matter how elaborate or impressive, a building is only as strong as its foundations, which if unstable can bring the entire structure to collapse.
In this context, the primary purpose of rational investigation is to identify a set of foundations which can serve as a solid support for knowledge. This purpose is present in everyday language when we ask: "how do you support that theory?", or "is that belief well founded?" It is here an implicit standard that knowledge should be supported, which in turn presupposes that knowledge can be supported. In other words, the foundational theory of knowledge is implicit, and often taken for granted in ordinary discourse, from school yard to the lecture hall.
The foundational theory of knowledge also implies a theory of criticism. In this view, criticism is conducted by checking to see whether a proposed idea is justified or unjustified relative to basic beliefs. If it is justified (or at least not in conflict) with basic beliefs, it is then knowledge. However, this theory of criticism implies a problem. If criticism is conducted relative to basic beliefs, and two parties in a discussion hold to different foundations, then there is an impasse where no agreement can be reached. There are logical limits to criticism.
The problem is deep. How are we to decide which basic beliefs are correct? If we try and check to see which basic beliefs are justified, then we will get different results depending which basic beliefs we choose. The choice among rival sets of basic beliefs would seem to be irrational: a leap of faith. If this problem cannot be solved, then it would seem that rational discussion among people who choose to make a different leap of faith is impossible, and no particular leap of faith can be better than any other, each is an arbitrary choice and all knowledge is relative.
To respond to this problem, it is often claimed that particular basic beliefs are self-evident, and it is a common claim when challenged upon basic beliefs that they are in no need of defence, because they are self-evident, and beyond question. Therefore, it is often believed that the holy grail of foundationism, and rational philosophy, is to identify a set of basic beliefs which are self-evident. It is supposed that this would defeat the relativist, who would point to the unjustifiability and arbitrariness of our foundations.
I can only wonder though why self-evidence should be considered in such esteem.It is an all too common occurrence, that different people come to varied and often contradictory conclusions regarding what is self-evident, in much the same way as they do regarding ordinary evidence. I am not aware of by what means self-evidence is elevated to that infallible plane which seems to make it such a desirable goal, or what relation to the truth which it is supposed that self-evidence possesses, or is that supposed to be self-evident?
It is also a puzzle to me that foundationists should honour self-evidence with such esteem. To me, the prime concern of rational investigation is the truth, which is objective i.e. what is true for one is true for all, and yet to also be concerned with self-evidence is puzzling, as it is little other than an expression of subjective confidence. I think foundationism is mistaken, and places unnecessary limits upon criticism. The search for a solid foundation for knowledge would destroy rationality, and not strengthen it as intended.