Since i view postmodernism as a logical, inevitable idea which will certainly die or mutate into something else.
it's a huge topic with a thousand points of entry. but we're still living in the age of postmodernism and pomo, for all intents and purposes, is the mental environment of our era. because we live in it, our minds are constantly reacting to it and interacting with the assumptions we don't even realize are there.
General definition of postmodernism: "We believe that there is no absolute truth, excepting the truth that there is no absolute truth."
Think about that, and the general mental environment of our times, the media, and what lies beneath the irony and funny cynicism that you hear all around you and in the voices of your friends.
The dark ages were stagnant by our modern measures (experts will disagree and say that innovation went on; still, attitudes were more or less locked into place, religion was supreme, and science proceeded slowly.)
In the enlightenment, people rediscovered the classical influence of ancient greece, including science and architecture. they asked scientific questions and got answers. architecture and art generally adhered to rigid principles of beauty. something was either beautiful or it was not. just check the agreed-upon standards. innovation in the arts was seen as self-serving and was frowned upon.
classicism saw the birth of the novel. the emphasis, in literature, was on extreme realism.
romanticism was a turn away from the standards of classicism. poets explored inner feelings; the artist and his/her feelings became the object of beauty. whitman, wordsworth, keats: they turned thier eye inward. emotions were more important than reason. nature was deeply appreciated. this is still going on in popular music today. it's possibly the default for lyric-writing, and it seems that people who don't explore poetry seem to believe that for a poem to be good, it should follow the romantic ideal.
modernism was a formal move toward innovation, a reaction against all formalized conceptions of what constituted art and beauty. an underlying idea was that 'innovation is beautiful.' experimentation dominated. new was good. this new attitude held the movement together, but it itself was formalized, and so ended up being just a new kind of convention that people had to bust through.
relativity and existentialism, i believe, fall in the gap here, and both have a hand in the motives of both modernism and postmodernism. existentialism says that reality is meaningless and it's up to you to provide meaning. relativity suggests that there is no preferred frame of reference.
postmodernism seems to take the final step. postmodern art admits to itself that it is art, and cannot really be a container for unassailable beauty. it acknowledges that it doesn't have that power, due to the unknowable influence of the viewer or reader's perspective. it acknowledges that other minds are co-creators of the art that they're being fed. a self-consciousness intrudes. art can be seen constantly winking at you, acknowledging that it's manipulating your feelings. television commercials seemed to pick up on this in the mid 1990s.
When all truths are suspect, nothing is really true. it is harder to create a moral standard, since the ideas in the basement admit no truths that are absolute universally. rather than take a stand, and in the absence of solid ground to stand on, many people seem to adopt cynical attitudes as a way to shove off the questions. nothing is true, nothing is real, everything is relative, therefore nothing really matters in any measurable way. for some, irony and cynicism and sarcasm seem to function as a mask to hide the vacuum within. a tacit ackowlegement that no one knows anything anymore, but we're going to try to laugh (nervously) about it until we figure something out.