D1 · The Perceptron
Two Gates Fall, One Does Not
Build AND and OR out of one neuron, and prove in two ways that XOR cannot be built that way.
Two inputs, each 0 or 1. Four possible examples. These are the smallest learning problems there are. AND fires only when both are 1. OR fires when at least one is. XOR fires when exactly one is. Two of these a single neuron solves in seconds. The third held the field back for years.