The word abstract strictly speaking means to separate or withdraw something from something else. In that sense applies to art in which the artist has started with some visible object and abstracted elements from it to arrive at a more or less simplified or schematised form. Term also applied to art using forms that have no source at all in external reality. These forms are often, but not necessarily, geometric. Some artists of this tendency have preferred terms such as Concrete art or non-objective art, but in practice the word abstract is used across the board and the distinction between the two is anyway not always obvious. A cluster of theoretical ideas lies behind abstract art. The idea of art for art's sake—that art should be purely about the creation of beautiful effects. The idea that art can or should be like music—that just as music is patterns of sound, art's effects should be created by pure patterns of form, colour and line. The idea, derived from the ancient Greek philosopher Plato, that the highest form of beauty lies not in the forms of the real world but in geometry. The idea that abstract art, to the extent that it does not represent the material world, can be seen to represent the spiritual. In general abstract art is seen as carrying a moral dimension, in that it can be seen to stand for virtues such as order, purity, simplicity and spirituality. Pioneers of abstract painting were Kandinsky, Malevich and Mondrian from about 1910-20. A pioneer of abstract sculpture was the Russian Constructivist Naum Gabo. Since then abstract art has formed a central stream of modern art.