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We have now come close to our question: What is the subject of fuzzy logic? There
is no doubt that independently of whether we consider pure fuzzy logic, applied
fuzzy logic, or practical fuzzy logic, the subject of all fuzzy logic is the so-called
vagueness, be it vagueness of individual objects, classes, relations, structures, sys-
tems, processes, or actions. Fuzzy logic is a scientific conceptualization of vague-
ness and a methodology of how to cope with it. But what is vagueness?
2.3
The Nature of Vagueness
Is vagueness something objective that exists in the world out there, or is it some-
thing subjective that mirrors our uncertainty about objects and occurrences? There
are those who maintain that it is something subjective, and those who say the oppo-
site. To answer this question, it is expedient to differentiate between four types of
vagueness:
1. Linguistic vagueness,
2. Epistemic vagueness,
3. Semantic vagueness,
4. Ontic vagueness.
As regards linguistic vagueness, phenomena such as unclarity of meaning, ambi-
guity, and polysemy are in principle remediable. They are not genuine vagueness.
The remaining class of genuinely vague linguistic expressions such as “bald” and
“young” which cannot be made precise, corresponds to ontic vagueness that will be
discussed below.
Epistemic vagueness is in fact subjective uncertainty due to lack of information
and knowledge. It is not genuine vagueness.
Semantic vagueness, or vague reference, is the representational vagueness, i.e.,
the unclear representation of an object y by a word or picture x when there is a rela-
tion of the form Repr
such that xrepresentsy . Examples are blurred pictures.
Semantic vagueness as vagueness of reference is a vague relation, and as such, it
also belongs to the following type of vagueness.
Ontic vagueness is the prototypical vagueness and the source of all other types
of genuine vagueness. 1 It concerns the vagueness of individual objects, classes,
relations, and states of affairs in 'the world out there'. Are there really such vague
entities? And what does their vagueness look like?
Concisely, we cannot know how things are 'in themselves' irrespective of whether
or how they are perceived, recognized, or represented, i.e., of the perspective from
which they are viewed ([8], ch. 23). The world looks different depending on what
glasses we put on. We may therefore be tempted to take the position that “we shall
never know whether there is vagueness in the world out there and whether objects
or states of affairs can be vague” . To assert or to deny vagueness in the world, will
remain an ontological postulate in any case. From a practical perspective, however,
(
x
,
y
)
1
The adjective “ontic” means “concerning the being”. It originates from the Greek term o ν
(on) that derives from the present participle of the Greek verb ειναι (einai) for “to be”.
 
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