Instinct - literally: natural drive - usually refers to the unknown, internal foundations ("drives") of the behavior of an animal as perceived by the observer. Since the Middle Ages, terms like instinct, drive, impulse, and others have been used, but not precisely defined. Instincts were initially regarded as a divine gift, the exact analysis of which was thought to be beyond the human mind, drawing on the thoughts of philosophers from ancient Greece. It was not until the 19th century, following advancements in the fields of anatomy and neurology, that a more pragmatic approach to the phenomenon of innate behavior became possible. The author and zoologist Heinrich Ernst Ziegler (1858-1925) supported the reflex chain theory in 1904 and stated that reflexes and instincts are based on inherited pathways of the nervous system.