In order to know anything, one’s faculties have to be generally telling the truth.
The various things we know come from a multiplicity of sources. I know I am warm because I feel it. I know there are billions of people in the world because I’ve been told so. I know I ate breakfast this morning because I remember it. I know I have feet because I see and feel them. I know that 2+3=5 because it is intuitively obvious.
Notice that in each case there is an experience which causes me to believe these things. Or in some of these cases, perhaps the experience just is my belief. But we are plainly “wired” to acquire different types of knowledge by means of different aspects of our cognitive faculties. Some of our knowledge comes through our senses, some through memory, some by reasoning, some by believing what we’re told.
One could try to maintain that all knowledge shares a single foundation; rationalists have claimed that though our knowledge comes from various sources, all these sources require the underwriting of reason. This claim is easily challenged, however.
Regardless, one thing is clear: We cannot have knowledge unless our faculties are generally telling us the truth.
The reason for this is that beliefs that just happen to be true do not constitute knowledge. Epistemologists agree that 1) one cannot know something without believing that thing, 2) one cannot know something unless that thing is true, and 3) the conjunction of this belief and truth cannot be just a happy coincidence. I cannot know that I have feet unless 1) I believe I have feet, 2) I really do have feet, and 3) it isn’t just a coincidence that I believe this thing which happens to be true; if our faculties are telling us something, what they are telling us only gives us knowledge if those faculties largely tell us the truth. (This explains the recent popularity of “Reliabilism” in Epistemology.)
So one can sensibly believe one knows anything at all only if one trusts that at least that aspect of one’s facilities are generally reliable—i.e., one trusts that those faculties generally deliver true rather than false beliefs.
The use of the word ‘trust’ here is not accidental.