  Alternative set theory was created by the Czech mathematician Petr Vop\v enka
in 1979 as an alternative to Cantor's set theory. Vop\v enka criticised
Cantor's approach for its loss of correspondence with the real world.
Alternative set theory can be partially axiomatised and regarded as a
nonstandard theory of natural numbers. However, its intention is much wider. It
attempts to retain a correspondence between mathematical notions and phenomena
of the natural world. Through infinity, Vop\v enka grasps the phenomena of
vagueness. Infinite sets are defined as sets containing proper semisets, i.e.
vague parts of sets limited by the horizon. The new interpretation extends the
field of applicability of mathematics and simultaneously indicates its limits.
This incidentally provides a natural solution to some classic philosophical
problems such as the composition of a continuum, Zeno's paradoxes and sorites.
Compared to strict finitism and other attempts at a reduction of the infinite
to the finite Vop\v enka's theory reverses the process: he models the finite in
the infinite.
