Beauty is studied as part of aesthetics, culture, social psychology and sociology. He worked in the fields of ethics, aesthetics, and ordinary language philosophy. He develops his philosophy over four books covering epistemology, ontology, aesthetics, and ethics. Many mathematicians talk about the "elegance" of mathematics, its intrinsic aesthetics and inner beauty. Aztec philosophers developed theories of metaphysics, epistemology, values, and aesthetics. Wall states that unpredictability is key to modern aesthetics. Schiller wrote many philosophical papers on ethics and aesthetics. Other considerations include cost, constructability, safety, aesthetics and sustainability. Kay stated that the group's visual aesthetics are important. They give at least some sense of the aesthetics of Greek painting. Johnson did not attempt to create schools of theories to analyse the aesthetics of literature. Kant published other important works on ethics, religion, law, aesthetics, astronomy, and history. The goth subculture has associated tastes in music, aesthetics, and fashion. It also is increasingly concerned with the brain basis for musical aesthetics and musical emotion. Functional tattoos are used primarily for a purpose other than aesthetics.