Medical knowledge as well as clinical practice are characterized by inescapable uncertainty. There are many reasons this is the case, but foremost among them is that almost everything in medicine is inevitably vague, be it something linguistic such as the term "illness", or something extra-linguistic such as the condition referred to as illness. If we ask ourselves, then, what the term "illness' means exactly, on the one hand; and how we may precisely delimit the condition ">illness, on the other; we shall recognize that to answer these and similar questions requires specific methods that enable us to adequately cope with vagueness. As we shall see below, fuzzy logic provides us with just such methods.