 But these criteria are not enough to provide the needed reassurance. First, we lack assurance that most proposed category systems meet even these minimal conditions. As mentioned above, Aristotle drew out his categories largely by considering the types of question that could be asked (and the types of answer appropriate to them). It is difficult to know, however, how one can be assured that all kinds of questions have been surveyed, and so difficult to know that an exhaustive list of categories has been offered – a point Aristotle does not attempt to demonstrate (Ackrill 1963, 80–81). Indeed, the fact that Aristotle provides different lists of categories in different places suggests that he did not consider his list final and exhaustive. Similarly, Kant’s system of categories can be thought to be exhaustive only as long as the list of forms of judgment from which he derives them exhausts the possible forms of judgment – but we have reason to think this is not so (Körner 1955, 50). Johansson, as we have seen, instead uses the method of successive abstraction. But it is not clear how following such a method could ensure either that the categories thereby distinguished are exhaustive (how do we know we have considered something of each highest kind if we do not yet know what the highest kinds are?) or even mutually exclusive.