There is no clear point in time when post-modernism began, nor is there a clear definition of what it is. In fact there is no single position which is 'the' post-modern position, as it covers a range of different responses to the modern world.
The term 'post-modernism' was first used in the late 1930s by the English historian Arnold Toynbee. It is a particular phenomenon of French culture. French philosophy, which is very different from Anglo-American, lays particular stress on the French language and the power of texts, which is clearly in evidence in post-modernism. Post-modernism may be connected with the student riots in Paris in 1968 and the Marxism and anarchist sentiments that lay beneath French culture at the time.
To the post-modern mind, context and perspective are everything. There is more than one form of post-modernism, but at least in some versions the reality we talk about and inhabit is one that we construct, that can be radically altered and one that has no fixed points of reference. In post-modernism there is no good or bad art, only the appreciation and interpretation each individual brings to what he or she sees. Thus, a painting using elephant dung that won the 'Turner Prize for art in Britain, a pile of bricks that was exhibited at the Tate Gallery and a sheep preserved in formaldehyde that was regarded as one of the great British modern works of art are all considered to rank equally with the paintings of Turner, Constable and Picasso.