Sir John Carew Eccles AC FRS FRACP FRSNZ FAA (27 January 1903 – 2 May 1997) was an Australian neurophysiologist and Philosopher who won the 1963 Nobel Prize in Physiology or Medicine for his work on the synapse. He shared the prize with Andrew Huxley and Alan Lloyd Hodgkin.

Eccles was born in Melbourne, Australia. He grew up there with his two sisters and his parents: william and Mary Carew Eccles (both teachers, who home schooled him until he was 12). He initially attended Warrnambool High School (now Warrnambool College) (where a science wing is named in his honour), then completed his final year of schooling at Melbourne High School. Aged 17, he was awarded a senior scholarship to study Medicine at the University of Melbourne. As a medical undergraduate, he was never able to find a satisfactory explanation for the interaction of mind and body; he started to think about becoming a neuroscientist. He graduated (with first class honours) in 1925, and was awarded a Rhodes Scholarship to study under Charles Scott Sherrington at Magdalen College, Oxford University, where he received his Doctor of Philosophy in 1929.

In 1937 Eccles returned to Australia, where he worked on military research during World War II. During this time Eccles was the Director of Kanematsu Institute at Sydney Medical School, he and Bernard Katz gave research lectures at the University of Sydney, strongly influencing its intellectual environment. After the war, he became a professor at the University of Otago in New Zealand. From 1952 to 1962 he worked as a professor at the John Curtin School of Medical Research (JCSMR) of the Australian National University. The Eccles Institute of Neuroscience is headquartered in a new wing of the JCSMR building, constructed with the assistance of a $63M grant from the Commonwealth Government and completed in March 2012.

Apart from these seminal experiments, Eccles was key to a number of important developments in neuroscience. Until around 1949, Eccles believed that synaptic transmission was primarily electrical rather than chemical. Although he was wrong in this hypothesis, his arguments led him and others to perform some of the experiments which proved chemical synaptic transmission. Bernard Katz and Eccles worked together on some of the experiments which elucidated the role of acetylcholine as a neurotransmitter in the brain.

In the early 1950s, Eccles and his colleagues performed the research that would lead to his receiving the Nobel Prize. To study synapses in the peripheral nervous system, Eccles and colleagues used the stretch reflex as a model, which is easily studied because it consists of only two neurons: a sensory neuron (the muscle spindle fibre) and the motor neuron. The sensory neuron synapses onto the motor neuron in the spinal cord. When a current is passed into the sensory neuron in the quadriceps, the motor neuron innervating the quadriceps produced a small excitatory postsynaptic potential (EPSP). When a similar current is passed through the hamstring, the opposing muscle to the quadriceps, an inhibitory postsynaptic potential (IPSP) is produced in the quadriceps motor neuron. Although a single EPSP was not enough to fire an action potential in the motor neuron, the sum of several EPSPs from multiple sensory neurons synapsing onto the motor neuron can cause the motor neuron to fire, thus contracting the quadriceps. On the other hand, IPSPs could subtract from this sum of EPSPs, preventing the motor neuron from firing.

He was appointed a Knight Bachelor in 1958 in recognition of services to physiological research.

He won the Australian of the Year Award in 1963, the same year he won the Nobel Prize.

In 1964 he became an honorary member to the American Philosophical Society, and in 1966 he moved to the United States to work at the Institute for Biomedical Research in Chicago. Unhappy with the working conditions there, he left to become a professor at the University at Buffalo from 1968 until he retired in 1975. After retirement, he moved to Switzerland and wrote on the mind-body Problem.

In The Understanding of the Brain (1973), Eccles summarises his philosophy: "Now before discussing brain function in detail I will at the beginning give an account of my philosophical position on the so-called 'brain-mind problem' so that you will be able to relate the experimental evidence to this philosophical position. I have written at length on this philosophy in my book Facing Reality. In Fig. 6-1 you will be able to see that I fully accept the recent philosophical achievements of Sir Karl Popper with his concept of three worlds. I was a dualist, now I am a trialist! Cartesian dualism has become unfashionable with many people. They embrace monism to escape the enigma of brain-mind interaction with its perplexing problems. But Sir Karl Popper and I are interactionists, and what is more, trialist interactionists! The three worlds are very easily defined. I believe that in the classification of Fig. 6-1 there is nothing left out. It takes care of everything that is in existence and in our experience. All can be Classified in one or other of the categories enumerated under Worlds 1, 2 and 3.

In 1981, Eccles became a founding member of the World Cultural Council.

In 1990 he was appointed a Companion of the Order of Australia (AC) in recognition of Service to science, particularly in the field of neurophysiology. He died in 1997 in Tenero-Contra, Locarno, Switzerland.

Eccles had nine children. Eccles married Irene Eccles in 1928 and divorced in 1968. After his divorce in 1968, Eccles married Helena Táboríková; a fellow neuropsychologist and M.D. of Charles University. The two often collaborated in research and they remained married until his death. Eccles died on 2 May 1997 in his home of Contra, Switzerland. He was buried in Contra, Switzerland.

"And what is World 3? As shown in Fig. 6-1 it is the whole world of culture. It is the world that was created by man and that reciprocally made man. This is my message in which I follow Popper unreservedly. The whole of language is here. All our means of communication, all our intellectual efforts coded in books, coded in the artistic and technological treasures in the museums, coded in every artefact left by man from primitive times—this is World 3 right up to the present time. It is the world of civilisation and culture. Education is the means whereby each human being is brought into relation with World 3. In this manner he becomes immersed in it throughout life, participating in the heritage of mankind and so becoming fully human. World 3 is the world that uniquely relates to man. It is the world which is completely unknown to animals. They are blind to all of World 3. I say that without any reservations. This is then the first part of my story.