Wolff is interested not just of truth and probability, but of different methods for discovering true and probable statements, that is, scientific and philosophical methodology. Now, as we have seen, Wolff has made the bold statement that logic is all that one requires for methodology, and at least when it comes to truth, logic is essentially based on demonstrations. This rather stringent description is easier to understand, once we remember that Wolff would accept also empirical statements as premisses of demonstration.
What I find remarkable is that Wolff mentions two different methodologies: a posteriori and a priori. In all external appearances, we are moving towards Kant, yet, there are still crucial differences. While for Kant a posteriori would mean all knowledge based on experience, Wolff limits the range of a posteriori to mere experiences. Thus, when Kant says that a posteriori knowledge cannot be universal, his statement is far stronger than Wolff's. Indeed, Wolff would verbally accept Kant's statement, but it would mean something less than with Kant – it would be just tautology because individual experiences are always singular.
Now, if a posteriori has no other meaning for Wolff, but individual experiences, a priori must then get everything else, that is, the class of a priori contains all bits of knowledge requiring demonstration. Now, demonstrations as meant by Wolff here can be either direct or indirect, while a priori can be either intuitive or discursive. Intuitive a priori has a nice paradoxical feeling to it, somewhat reminiscent of later and more famous synthetic a priori, yet, is not completely identical with it. Intuitive apriority characterises all those truths that can be directly intuited as true, just by carefully reflecting on the elements of the supposed truth. The class of such truths contains analytic axioms of logic, but also immediate truths about causes and effects, which Kant would have classified as synthetic a priori.
The final class of knowledge statements contains then discursive apriorities, that is, demonstrated truths. As one should remember, these demonstrations could have reliable experiences as premises. Furthermore, Wolff also included inductions as just one modification of syllogistic reasoning. Wolff's class of discursive apriorities contains then a lot that Kant would have classified as a posteriori, and in fact is probably the largest of the three classes.
Within this threefold classification Wolff places his main methodological considerations: when is experience to be relied upon, when can we say that something is a cause, when can we use experiences to draw general conclusions etc. I shall refrain from going into intricate details, and instead, I shall next time look at how Wolff thinks scientific conclusions should be presented.