Taiwanese has been analyzed either as a suppletive form of le or an equivalent of a higher abstract verb 'YOU', that asserts the existence of an event or a state. The latter seems plausible. However, when we carefully examine the various semantic functions of u, we find that they lack the constancy which would allow them to be united as a higher existential 'U'. This does not mean that there are various separate u's. We propose there is only one morpheme u. The 'major' various semantics for u, i.e. 'expectation', 'emphasis', 'perfectivity' and 'existence-possession', are, metaphorically connected. The links between them are reflected in a series of ambiguous sentences with u, in which an identical form of utterance may represent different meanings according to different contexts of use.