History of Western Philosophy - Bertrand Russell [337]
He goes on to explain that substances do not act on each other, but agree through all mirroring the universe, each from its own point of view. There can be interaction, because all that happens to each subject is part of its own notion, and eternally determined if that substance exists.
This system is evidently just as deterministic as that of Spinoza. Arnauld expresses his horror of the statement (which Leibniz had made): 'That the individual notion of each person involves once for all everything that will ever happen to him.' Such a view is evidently incompatible with the Christian doctrine of sin and free will. Finding it ill received by Arnauld, Leibniz carefully refrained from making it public.
For human beings, it is true, there is a difference between truths known by logic and truths known by experience. This difference arises in two ways. In the first place, although everything that happens to Adam follows from his notion, if he exists, we can only ascertain his existence by experience. In the second place, the notion of any individual substance is infinitely complex, and the analysis required to deduce his predicates is only possible for God. These differences, however, are only due to our ignorance and intellectual limitations; for God, they do not exist. God apprehends the notion of Adam in all its infinite complexity, and can therefore see all true propositions about Adam as analytic. God can also ascertain a priori whether Adam exists. For God knows His own goodness, from which it follows that He will create the best possible world; and He also knows whether or not Adam forms part of this world. There is therefore no real escape from determinism through our ignorance.
There is, however, a further point, which is very curious. At most times, Leibniz represents the Creation as a free act of God, requiring the exercise of His will. According to this doctrine, the determination of what actually exists is not effected by observation, but must proceed by way of God's goodness. Apart from God's goodness, which leads Him to create the best possible world, there is no a priori reason why one thing should exist rather than another.
But sometimes, in papers not shown to any human being, there is a quite different theory as to why some things exist and others, equally possible, do not. According to this view, everything that does not exist struggles to exist, but not all possibles can exist, because they are not all 'compossible'. It may be possible that A should exist, and also possible that B should exist, but not possible that both A and B should exist; in that case, A and B are not 'compossible'. Two or more things are only 'compossible' when it is possible for all of them to exist. Leibniz seems to have imagined a sort of war in the Limbo inhabited by essences all trying to exist; in this war, groups of compossibles combine, and the largest group of compossibles wins, like the largest pressure group in a political contest. Leibniz even uses this conception as a way of defining existence. He says: 'The existent may be defined as that which is compatible with more things than is anything incompatible with itself'. That is to say, if A is incompatible with B, while A is compatible with C and D and E, but B is only compatible with F and G, then A, but not B, exists by definition. 'The existent,' he says, 'is the being which is compatible with the most things.'
In this account, there is no mention of God, and apparently no act of creation. Nor is there need of anything but pure logic for determining what exists. The question whether A and B are compossible is, for Leibniz, a logical question, namely: Does the existence of both A and B involve a contradiction? It follows that, in theory, logic can decide the question what group of compossibles is the largest, and this group consequently will exist.
Perhaps, however, Leibniz did not really mean that the above was a definition of existence.