McCulloch–Pitts neuron
The 1943 mathematical nerve cell: binary inputs, a threshold, and the proof that a network of them can compute any logical proposition.
Warren McCulloch was a neurophysiologist, Walter Pitts a self-taught logician in his early twenties whom McCulloch had taken into his home. Their 1943 paper A Logical Calculus of the Ideas Immanent in Nervous Activity made a drastic simplification of the nerve cell: it either fires or it doesn't, so it is a Boolean variable. Sum the inputs, compare against a threshold, fire if the threshold is reached. Inhibitory inputs have an absolute veto. No weights, no learning, no time beyond discrete steps.
The proof was the point. From such elements you can build and, or and not — and therefore any proposition in propositional logic. Attach the network to an external tape and it becomes equivalent to a Turing machine. The brain was thereby described as a logical machine, which was a bold thing to claim in 1943 and remains one.
Von Neumann read the paper and cited it — it is the only reference in the First Draft of a Report on the EDVAC, and his "organs" in that machine description are McCulloch and Pitts's elements in disguise. Weights and learning came later, with Hebb in 1949 and Rosenblatt's perceptron in 1958. Pitts burned his unpublished work after a rupture with Norbert Wiener and drank himself to death in 1969, aged forty-six.