Under the principle of interaction simultaneity, the neural firings connected by interaction via the synapse are considered to be simultaneous. The time parameter thus constructed is called the proper time. In reference to the firing event of a particular neuron A, the presynaptic firings that produced EPSP in neuron A, and the postsynaptic firings that are influenced by the firing of neuron A, are considered to be simultaneous. The principle of interaction simultaneity is derived from the assumption of causality.