The main argument for foundationalism is called the regress argument. It’s an argument from elimination. With regard to every justified belief, B1, the question arises of where B1’s justification comes from. If B1 is not basic, it would have to come from another belief, B2. But B2 can justify B1 only if B2 is justified itself. If B2 is basic, the justificatory chain would end with B2. But if B2 is not basic, we need a further belief, B3. If B3 is not basic, we need a fourth belief, and so forth.