<P> The term "visceral" is contrasted with the term "parietal", meaning "of or relating to the wall of a body part, organ or cavity" The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides . </P> <P> Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal), and to describe more abstract "parts" of an interconnected whole (e.g. his philosophical works, taken as a whole, are referred to as the "organon"). </P> <P> Some alchemists (e.g. Paracelsus) adopted the Hermetic Qabalah assignment between the seven vital organs and the seven classical planets as follows: </P> <Table> <Tr> <Td> Planet </Td> <Td> Organ </Td> </Tr> <Tr> <Td> Sun </Td> <Td> Heart </Td> </Tr> <Tr> <Td> Moon </Td> <Td> Brain </Td> </Tr> <Tr> <Td> Mercury </Td> <Td> Lungs </Td> </Tr> <Tr> <Td> Venus </Td> <Td> Kidneys </Td> </Tr> <Tr> <Td> Mars </Td> <Td> Gall bladder </Td> </Tr> <Tr> <Td> Jupiter </Td> <Td> Liver </Td> </Tr> <Tr> <Td> Saturn </Td> <Td> Spleen </Td> </Tr> </Table>

Which characteristic of life is the digestive system