Philosophy especially examines what we claim to know and the presuppositions behind our knowledge claims. It does this to determine if our beliefs are simply opinions, and therefore possibly false, or if they are more than this. When persons are asked "why do you believe that?" they are pressed to give an account, or a reason, for their conclusion. This comes in the form of premises, supporting beliefs, that are intended to show that the conclusion is more than just a random opinion but instead is entailed by other beliefs the person claims to have knowledge about. These beliefs can also be questioned, and this process of looking at assumptions, or presuppositions, and testing them for meaning, can be called presuppositional thinking.
This process has led some philosophers to claim that there must be beliefs that can no longer be questioned, they are "properly basic." Now, we can speak about "properly basic" in a logical sense of being the most basic beliefs held by a worldview, the beliefs that support or are presupposed by all other beliefs in that worldview. We can also speak about "properly basic" in an epistemological sense of that which cannot be questioned because it makes questioning possible. In my last post, "Reason: The laws of thought," I make the claim that only the laws of thought hold this position. These laws of thought can be applied to the logically most basic beliefs of any given worldview to test them for meaning.
This process has also been rejected by some philosophers under the title "foundationalism." The argument is that beliefs do not come to us in this order (say, like a ladder with upper steps resting on lower steps), but instead more horizontally. They are indeed connected by consistency, but they might loop around and connect to each other in a way that is formally circular but not in a problematic way (according to proponents of this approach).
I'm not too particular about names, nor need we identify all the brands of foundationalism and coherentism here. The relevant point is that even the coherentist, or anyone else who rejects foundationalism, must respond to the question "why do you believe that?" because the alternative is that he/she is simply offering us an opinion which might turn out to be false.
So when we give an answer to a question we are looking for knowledge, not opinion. Knowledge is distinguished from opinion because it has justification (rational justification) showing it cannot be mistaken. Identifying knowledge with certainty sometimes elicits the response "we never have certainty!" But this is just the philosophical viewpoint called skepticism (knowledge is not possible). When asked "why do you believe that?" the skeptic must respond or concede that skepticism is simply one opinion with nothing to attract us to it and it might be false. This concession is the same as remaining silent. Therefore, we can summarize our options as: i) seeking knowledge; ii) remaining silent.