One of the most outrageous bits of Wolff's metaphysics was his apparent attempt to deduce principle of sufficient reason from the principle of non-contradiction. Some readers have even declared that Wolff has thus Leibnizian distinction between logical and empirical truths: Leibniz said that logical truths were based on the principle of non-contradiction and empirical truths on the principle of sufficient reason, so Wolff's deduction apparently showed empirical truths to be logical. This is clearly a mistaken reading, because Wolff still accepts the Leibnizian distinction and use of the two principles as criteria. Indeed, Leibniz had merely said that logical truths were necessarily true, because their opposites were contradictory, while the empirical truths require some previous explanation and were thus not necessary. Wolff just added that the latter criterion was itself necessarily true and in need of no further explanation.
Although Wolff's proof of the principle of sufficient reason does not reduce all truths into logical truths, the proof itself was rather unconvincing and based on a very ontological reading of the principle of non-contradiction. Now, Wolff apparently felt the need to justify the principle once again.
First of all, Wolff notes that actually the principle as such requires no proof, because no philosopher and indeed no human being would truly doubt it – or at least if someone says he doubts, he is still bound to use the principle unconsciously, when asking causes of events and motives of actions. I already noted that Wolff had used in the original book a strategy where the principle of sufficient reason was justified as a necessary presupposition of us having coherent experience. Here Wolff is then suggesting that the principle is somehow natural to human mind. Both strategies have a Kantian feel, especially if one combined them: we couldn't have experience without using the principle of sufficient reason, thus, the principle must be ingrained in us.
Still, there is a difference. While Kant apparently speaks determinedly of causality as a necessary presupposition of experience, Wolffian ground is something in thing A that explains something else in thing B, where A and B might be also the same thing. Examples Wolff uses clearly show that in addition to causal influences he is also thinking of motives as possible grounds. Wolff is thus not saying that ”every event has a cause” would be true of all experience, but instead the more general statement ”every event either has a cause or is a motivated action”.
Wolff also explains his original proof of the principle through the simile of scales: if two sides of the scales are evenly balanced, the scales does not tip to either direction, and if the scales do tip, something must have been added or taken away to change the balance. In effect, this simile confirms my interpretation of the proof: the different possibilities, as it were, compete with one another for the chance of actualisation and because of their opposition, they would remain eternally in a state of null actuality, unless something came and changed the scales in favour of one possibility. What appears different in Wolff's new account is the admission that in case of human actions grounds or motivations might not completely determine the action: human being has still the opportunity to choose what motive he is going to emphasize – we shall later see what effects this admission has on Wolff's psychology.
Wolff also presents a completely new line of defense for the principle. In essence, he outlines three possibilities: firstly, the principle of sufficient reason might hold always, it might hold never, or it might hold sometimes, but not always. Somewhat hastily, Wolff concludes that the second option cannot be true, because experience tells us that at least some events have had a preceding ground (remember that Hume's criticism of causality was still to come). How about then the third possibility? Wolff suggests that if there were no possibility to actually say when something has a ground and when not, then we would actually land back to the second possibility. Thus, there must always be ground telling whether there is a ground or not – and then we are actually in the first option that the principle holds in all cases.
Wolff's argument is based on his fault of not underlining yet another form of ”ground”. This ground is more of an explanation based on ”form” or structure of events: just like we can justify the proposition that a figure has angles adding up to 180 degrees by its being triangle, similarly events might have some structural features that either make the principle to apply them or not. Indeed, Wolff has not managed to justify the principle, but he has noted the possibility of a third ontological position between full determinism and full indeterminism, which we might call restricted or controlled indeterminism. That is, if we do not want to admit that all events are deterministically caused, we do not have to take everything to be indeterministic, if we suppose that indeterminism applies only within some restricted area of events, which does not hinder the determinism outside this area. This possibility seems interesting, firstly, because it appears to fit in with current state of physics (the area would have to be defined in terms of e.g. size of entities involved). Secondly, it might even be compatible with Kantian notion of deterministic causality as a presupposition of experience, that is, indeterministic causality might not hinder the possibility of experience, if it was restricted to some area that was a) rarely seen in experience and b) controlled in the sense that effects in that area would have no real effect outside that area.
So much for the principle of sufficient reason, next time I'll wrap up with Wolff's comments on the rest of ontology.