In 1943 the neurophysiologist Warren S. McCulloch and the logician Walter Pitts published “A Logical Calculus of the Ideas Immanent in Nervous Activity” in the Bulletin of Mathematical Biophysics (volume 5, pages 115 to 133). It was the first attempt to describe a biological neuron in precise mathematical terms. Only the year is documented for the publication date.
The paper treats each neuron as a simple all-or-none switch: it either fires or it does not, depending on whether the signals reaching it cross a threshold. The authors proved that networks built from these idealized neurons can represent any statement of propositional logic. In other words, a brain made of such units could, in principle, carry out any well-defined logical computation.
This was a landmark because it connected the biology of the brain to the formal logic underlying computing machines. The idea that thinking might be reduced to networks of threshold units became one of the founding assumptions of artificial intelligence and the direct conceptual ancestor of every artificial neural network used today.