Omne possibile exigit existere. - Gottfried Wilhelm Leibniz

Now, since in the divine ideas there is an infinity of possible universes of which only one can exist, the choice made by God must have a sufficient reason which determines him to the one rather than to another. This reason can be found only in fitness, that is, in the degree of perfection contained in these worlds. For each possible has a right to claim existence in proportion to the perfection it involves. Thus nothing is entirely arbitrary.

Il y a deux labyrinthes fameux où notre raison s’égare bien souvent : l'un regarde la grande question du libre et du nécessaire, surtout dans la production et dans l'origine du mal ; l'autre consiste dans la discussion de la continuité et des indivisibles qui en paraissent les éléments, et où doit entrer la considération de l'infini.

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.