In short the "modern" had ideas - single unifying truths - this is what
the post-modern lacks. <- this sentence is offering a single definition
of post-modernism and therefore contradicts itself - this irony is
generated by your question - which is part of modern methodology, that
simple questions can be answered with definitite/universal answers.
I think the cause of this phenomenon - the end of modernity includes the
ideas of the success/failure of the modern project- either stops its
progress or the idea of progress.
In philosophy - the end of philosophy - it merges with/into literature,
meaning becomes subjective.
In the Arts - the breakdown between high and low art. Art becomes
entertainment.
In Economics - capitalism is replaced by consumerism.
In politics the same - government by focus groups.
In science the realisation of limits which are more anthropological than
fundamental.
The conflation of science with technology - which hasn't produced
anything "new" for about a hundred years - just re-packages - for a new
consumer group.
Post-Script:
Post-modernity in universities is mainly in the lit-crit depts - and
this should not be confused with the general - post-modern condition.
Its *not liked* by the scientific community due to its subjectivism. (or
from any body which upholds universal values - including morals) However
these bodies in themselves demonstrate the post-modern condition in
their focus on popularism rather than truth. It is *liked* by
"outsiders" - Gays - Feminists - racially disadvantaged, new agers etc.
However these demonstrate counter "universal" ideas - so are "proto-
moderns"
IMO the clearest way into the subject is architecture, in post-modernity
the re-introduction of decoration which plays no part in upholding the
structure - which can disguise the underlying structure in a deliberate
(sometimes playful) way. This is easier to grasp than when the same kind
of thing goes on in literature.
Much as i've tried i think post-modern art is a not possible as art is
in essence a modern product. I'm not able to be a populist/ populiser -
but you might want to listen to a failed attempt at
www.jameswhitehead.org
--
James Whitehead