The Russian Constructivist painters Wassily Kandinsky and Kasimir Malevich and the sculptor Naum Gabo were pioneers of Non-objective art. It defines a type of abstract art that is usually, but not always, geometric and was inspired by the Greek philosopher Plato who believed that geometry was the highest form of beauty. Non-objective art may attempt to visualise the spiritual, and can be seen as carrying a moral dimension, standing for virtues like purity and simplicity. In the 1960s a group of American artists, including Sol LeWitt and Donald Judd, embraced the philosophy of Non-objective art. By creating highly simplified geometric art out of industrial materials they elevated these to an aesthetic level. Their work became known as Minimal art.