Fallibilism

The recognition that there are no authoritative sources of knowledge, nor any reliable means of justifying knowledge as true or probable.

To trust a source, you need a reason to trust it. But that reason is itself a source, so you need a reason to trust that. And so on, forever. There's no place the chain can legitimately stop. Any stopping point is just a place you decided to stop which means it's assumption, not justification.