Reason does not consider [its] hypotheses as first principles but as stepping stones to take off from, enabling it to reach the unhypothetical first principle of everything. Having grasped this principle, it reverses itself and, keeping hold of what follows from it, comes down to a conclusion.
Up and down that hill to fetch a pail of water that runs through your hands. On the move, we are.
(The quotation is from Plato's Republic, book 6, 511b; Grube-Reeve translation.)