Gott sah unendlich viele Welten als möglich vor sich; aber aus diesen unendlich vielen wählte er die wirkliche als die beste. - Gottfried Wilhelm Leibniz

We never have a full demonstration, although there is always an underlying reason for the truth, even if it is only perfectly understood by God, who alone penetrated the infinite series in one stroke of the mind.

Take the case (to use an easy example) of a river, carrying boats and communicating to them its own velocity, yet limited by their own inertia so that, all the rest being equal, the more heavily loaded will be carried more slowly. Hence it can be stated that the speed of the boats comes from the river, the slowness, from the load; the positive, from the force of the propelling agent, the privative, from the inertia of the propelled. Quite in the same manner it may be said that God contributes to the creatures their perfections, yet is limited by their receptivity. Thus all goods are due to the divine force; the evils, to the torpor of the creature.

There are two kinds of truths: those of reasoning and those of fact. The truths of reasoning are necessary and their opposite is impossible; the truths of fact are contingent and their opposites are possible.