see section 4
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A case in point is that of the realis - irrealis distinction.
These are terms widely used by various scholars and in various grammars. The core of the distinction is a desire to distinguish between real events and unreal ones.
That is, events that are or have happened versus events that did not or have not happened but which are possible, probably, hypothetically likely, or could have happened.
As can be seen from the list (which is by no means extensive) is
that irrealis notions cover a wide range of categories
while realis is a relatively simple affair.
The problem is that the term irrealis is used in grammatical descriptions in such a way that normally only a subset of irrealis notions is covered by presumed irrealis morphemes (see Bybee 1998 for a good description of the problem).
The problem is not just limited to morphemes that are called irrealis;
other types of morphemes, such as subjunctive or optative morphemes are affected in the same way.
Palmer (2001) devotes large portions of his discussion to the problem of how to link subjunctive with irrealis, for instance.
The immediate consequence is that it is a priori impossible to compare irrealis morphemes from one language to the next.
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