Because of the "all-or-none" character of nervous activity, events and the relations among them can be treated by means of propositional logic. It is found that the behaviour of every net can be described in these terms, with the addition of more complicated logical means for nets containing cycles; and that for any logical expression satisfying certain conditions, one can find in net behaving ever fashion it describes.

p. 115 - A logical calculus of the ideas immanent in nervous activity (1943)