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Walter Pitts (right) with Jerry Lettvin, co-author of the seminal cognitive science paper "What the Frog's Eye Tells the Frog's Brain" (1959)

Walter Pitts (23 April 1923 – 14 May 1969) was a logician who worked in the field of cognitive psychology. He proposed landmark theoretical formulations of neural activity and emergent processes that influenced diverse fields like cognitive sciences and psychology, philosophy, neurosciences, computer science, artificial neural networks, cybernetics and artificial intelligence. He is most remembered for having written along with Warren McCulloch, the seminal paper in scientific history called "A Logical Calculus of Ideas Immanent in Nervous Activity" (1943).

Pitts taught himself logic and mathematics and was able to read a number of languages including Greek and Latin. At the age of 12 he spent three days in a library reading Principia Mathematica and sent a letter to Bertrand Russell pointing out what he considered serious problems with the first half of the first volume. Russell was appreciative and invited him to study in the United Kingdom; although this offer was apparently not taken up, Pitts decided to become a logician.

He attended lectures at the University of Chicago, without registering as a student, and met Jerome Lettvin with whom he became good friends. In 1938 he met Rudolf Carnap by walking into his office and presenting him with an annotated version of Carnap's recent book on logic. Since Pitts did not introduce himself, Carnap spent months searching for him, and when he found him he obtained for him a menial job at the university. Pitts at the time was homeless and without income.

Later Warren McCulloch also arrived at the university, and in early 1942 invited Pitts, who was still homeless, together with Lettvin to live with his family. In the evenings McCulloch and Pitts collaborated. Pitts was familiar with the work of Gottfried Leibniz on computing and they considered the question of whether the nervous system could be considered a kind of universal computing device as described by Leibniz. This led to their seminal neural networks paper A Logical Calculus of Ideas Immanent in Nervous Activity.

In 1943 Lettvin introduced Pitts to Norbert Wiener at MIT, who had recently lost his "right-hand man". Their first meeting, where they discussed Wiener's proof of the ergodic theorem, went so well that Pitts moved to Boston to work with Wiener. In 1944 Pitts was hired by Kellex Corporation, part of the Atomic Energy Project.

In 1951 Wiener convinced Jerry Wiesner to hire some physiologists of the nervous system. A group was established with Pitts, Lettvin, McCulloch, and Pat Wall. Pitts wrote a large thesis on the properties of neural nets connected in three dimensions. Lettvin described him as "in no uncertain sense the genius of the group … when you asked him a question, you would get back a whole textbook". Pitts was also described as an eccentric, refusing to allow his name to be made publicly available. He refused all offers of advanced degrees or official positions at MIT as he would have to sign his name.

Wiener suddenly turned against McCulloch, on account of his wife Margaret Wiener who hated McCulloch, and broke off relations with anyone connected to him including Pitts. This sent Walter Pitts into 'cognitive suicide', a gradual but steep decline into social isolation, from which he never recovered. He burnt the manuscript on three dimensional networks and took little further interest in work. The only exception was a collaboration with Robert Gesteland which produced a paper on olfaction. Pitts died in 1969. The mathematical model of a neuron is today called as McCulloch–Pitts neuron. The theoretical formulation of the neural activity of the brain remains as the lasting legacy of Walter Pitts and Warren McCulloch to the Cognitive sciences.

• Warren McCulloch and Walter Pitts, A Logical Calculus of Ideas Immanent in Nervous Activity, 1943, Bulletin of Mathematical Biophysics 5:115-133.
• Warren McCulloch and Walter Pitts, On how we know universals: The perception of auditory and visual forms, 1947, Bulletin of Mathematical Biophysics 9:127-147.
• Howland, R., Jerome Lettvin, Warren McCulloch, Walter Pitts, and P. D. Wall, Reflex inhibition by dorsal root interaction, 1955, Journal of Neurophysiology 18:1-17.
• Wall, P. D., Warren McCulloch, Jerome Lettvin and Walter Pitts, Effects of strychnine with special reference to spinal afferent fibres, 1955, Epilepsia Series 3, 4:29-40.
• Jerome Lettvin, Humberto Maturana, Warren McCulloch, and Walter Pitts, What the Frog's Eye Tells the Frog's Brain, 1959, Proceedings of the Institute of Radic Engineers 47: 1940-1959
• Humberto Maturana, Jerome Lettvin, Warren McCulloch, and Walter Pitts, Anatomy and physiology of vision in the frog, 1960, Journal of General Physiology, 43:129--175
• Robert Gesteland, Jerome Lettvin and Walter Pitts, Chemical Transmission in the Nose of the Frog, 1965, J.Physiol. 181, 525-529.

• Cognitive science
• Warren McCulloch
• Jerome Lettvin

• Talking Nets: An Oral History of Neural Networks, Edited by James A. Anderson and Edward Rosenfeld, 1998. The interview with Jerome Lettvin discusses Walter Pitts.
• Pitts, Walter, MIT Encyclopedia of Cognitive Science