On the other hand, fever is a natural sign of disease. The changing pitch of a whistle is a natural sign of a fast traveling locomotive. A sign has to be interpreted, while a symbol has a meaning by itself. White, for instance, is a symbol of purity. A race starts when an audible sign is given.
Physiology is a branch of biology.
On the other hand, philosophy stands by itself, as does science. There are as many philosophical schools as there are definitions of wisdom, which is supposed to bring happiness or at least the acceptance of unhappiness. As the concrete components were being understood, philosophy was losing scope until finally it encompassed only the abstract aspects. Consider, however, that the Ph. Thus, a Ph.D means that the holder of such honor is highly qualified to impart knowledge.