“The third category certainly springs from the connection of the first and second, not, indeed, from their mere combination, but from a connection the possibility of which constitutes a concept that is a special category. For this reason the third category may not be applicable in instances in which the other two apply: _e.g._ _one_ year, _many_ years of future time, are real concepts, but the _totality_ of future years, that is, the collective unity of a future eternity, conceived as entire (so to say, as completed), is something that cannot be thought. But even in those cases in which the third category is applicable, it always contains something more than the first and the second taken separately and together, namely the _derivation_ of the second from the first, a process which is not always practicable. Necessity, for example, is nothing else than existence, _in so far as_ it can be inferred from possibility. Community is the reciprocal _causality_ of _substances_ in respect of their determinations. But that determinations of one substance can be produced by another substance, is something that we may not simply assume; it is one of those connections without which there could be no reciprocal relation of things in space, and therefore no outer experience. In a word, I find that just as the conclusion of a syllogism indicates, in addition to the operations of understanding and judgment in the premisses, _a special operation peculiar to reason_ ..., so also the third category is a special, and in part original, concept. For instance, the concepts, _quantum_, _compositum_, _totum_, come under the categories unity, plurality, totality, but a _quantum_ thought as _compositum_ would not yield the concept of totality unless the concept of the _quantum_ is thought as determinable through the _composition_, and in certain _quanta_, such as infinite space, that cannot be done.”[744]