Let reason work with the senses to find truth. Draw your first principles from what you observe — you have no other starting point. You must fall back on yourself. Even the universe itself and the God who guides it reaches out into all things, but then returns completely back to itself. Our minds should do the same thing. When your mind follows your senses out into the world, it should still remain master of them and of itself.
Let reason be encouraged by the senses to seek for the truth, and draw its first principles from thence: indeed it has no other base of operations or place from which to start in pursuit of truth: it must fall back upon itself. Even the all-embracing universe and God who is its guide extends himself forth into outward things, and yet altogether returns from all sides back to himself. Let our mind do the same thing: when, following its bodily senses it has by means of them sent itself forth into the things of the outward world, let it remain still their master and its own.