First assumptions are the removable building blocks of the world's possibilities.
If we logically reverse our process of re-creating the world back to our first assumptions, they are apparently neither the same, nor possible to reduce any further without at the same time replacing them by another set of first assumptions. If we replace our first assumptions with another set of first assumptions, but don't know it, we can't simply assume that we have replaced them, or else we must also know why. If humans say something but have no assumptions, then they didn't speak as-human. This is so because by definition all thought is based on such assumptions. As they change, or, more correctly, as we decide to change them, so do our belief systems.