The difference of individuals and universal properties has been recognized at least since the time of Aristotle. Indeed, it is obvious that the universal genus of horse is not an individual horse, although on some reading Plato had treated the genus just as one individual among others. Despite the familiarity of the distinction, it is quite hard to say what exactly differentiates universals from individuals.
Now, Thümmig suggests the rather curious definition that individuals are fully determinate in every way, while universals are still further determinable. The idea behind the strange definition is actually rather simple. Take some general class of things, such as vertebrates. Now, if we know that an animal is a vertebrate, we know something of it – at least that it has a vertebra. Still, many other characteristics of the said animal are completely undetermined by its being a vertebrate, for instance, whether it flies or not. Universal vertebrate is thus determined through this collection of properties shared by all vertebrates. This collection does still not determine any concrete individual, because a particular vertebrate has still some characteristics not included in the collection.
Similarly, all concrete individuals must be completely determined in respect of all possible characteristics (presumably there's an infinity of such possible characteristics). In other words, we cannot have an individual thing that would neither have a certain characteristic nor not have it: the individual must be determinately one or the other. Furthermore, nothing but a completed determination of possible characteristics could individuate a particular thing. One might object that it could still be possible that an individual is identifiable through some incomplete list of characteristics – for instance, George Washington can be plucked out from the rest of the humanity by him being the first president of United States, even if we didn't knew what he was called. But the objection forgets that in Wolffian philosophy we are allowed to look at other possible worlds. Thus, there could be another possible world where the first president of United States was a man called Thomas Jefferson, and the given description would not distinguish the two possible first presidents. Note that while an individual is determinate in all aspects, we might not be able to determine all its aspects.
Some universals and no individuals are then clearly indeterminate in some respect, but Thümmig's definition suggests also that all completely determinate things are individuals, but never universals. This is a far more uncertain proposition. Suppose for instance that we would know a particular rock and all its characteristics completely. Now, if we could then copy the rock and its exact characteristics, we would have two different individuals with the exactly same characteristics. In fact, the list of these characteristics would be completely determined - this was the presupposition - but it would also define a universal class containing several individuals (the two rocks).
Thümmig's definition thus clearly presupposes the idea that no two individuals could have a matching set of characteristics. This principle of the identity of indiscernibles originates actually from Leibniz, who according to a story once challenged courtiers to look for two exactly similar leaves just to prove the principle. Indeed, the principle might well be empirically sound, but as the thought experiment shows, it shouldn't be really accepted as an incontestable axiom of pure reason – and certainly it should not be hidden within a definition. Still, Thümmig's mistake is small when compared to what Baumgarten later did with the same notions – more on this later.
Next time we shall look on animal psychology.