An ornithopter (from Greek ornithos "bird" and pteron "wing") is an aircraft that flies by flapping its wings. Designers seek to imitate the flapping-wing flight of birds, bats, and insects. Though machines may differ in form, they are usually built on the same scale as these flying creatures. Manned ornithopters have also been built, and some have been successful. The machines are of two general types: those with engines, and those powered by the muscles of the pilot.

The Sanskrit epic Ramayana (4th Century BC) describes an ornithopter, the Pushpaka Vimana. The ancient Greek legend of Daedalus (Greek demigod engineer) and Icarus (Daedalus's son) and The Chinese Book of Han (19 AD) both describe the use of feathers to make wings for a person but these are not actually aircraft. Some early manned flight attempts may have been intended to achieve flapping-wing flight though probably only a glide was actually achieved. These include the flights of the 11th century monk Eilmer of Malmesbury (recorded in the 12th century) and the 9th century poet Abbas Ibn Firnas (recorded in the 17th century).Roger Bacon, writing in 1260, was also among the first to consider a technological means of flight. In 1485, Leonardo da Vinci began to study the flight of birds. He grasped that humans are too heavy, and not strong enough, to fly using wings simply attached to the arms. Therefore he sketched a device in which the aviator lies down on a plank and works two large, membranous wings using hand levers, foot pedals, and a system of pulleys.

Erich Walther von Holst (November 28, 1908 – May 26, 1962), was a German behavioral physiologist who was a native of Riga, Livonia and was related to historian Hermann Eduard von Holst (1841–1904). In the 1950s he founded the Max Planck Institute for Behavioral Physiology at Seewiesen, Bavaria.

Holst is remembered for his work with zoologist Konrad Lorenz (1903–1989) concerning the processes of endogenous generation of stimuli and of central coordination as a basis of behavioral physiology. This idea refuted the existing "reflex theory" which stated that this behavior was based on a chain of reflexes.

Holst postulated that the basic central nervous configuration consisted of a "cell" permanently producing endogenous stimulation, but prevented from activating its effector by another "cell" that also produced endogenous stimulation which contained an inhibition effect. This inhibiting "second cell" was influenced by the receptor, and stopped its inhibitory functionality precisely at the biologically right moment. In this fashion normal physiological stability was achieved.