Ostensible refers to the nature and the complicated structure of the matters that form everything that is ostensible. The ancient Greeks believed that everything was comprised of four pure elements—earth, water, air, and fire which could not exist in their pure forms. These elements were also used in describing people's psychology. Hippocrates used the four elements to explain the four humors found in the human body. According to this theory, the temperaments and the humor needed to be in balance with each other in order for a person to be well both mentally and physically. Although these ancient theories are no longer valid, it is possible to draw parallels between the four states of the matter agreed by the modern science and the four elements of the matter that were taken as the cornerstone of philosophy, science, and medicine for two hundred years.