.1 Logical Calculus of the Ideas Immanent in Jservous Activity 381 



that tlie difference is immaterial to our argument. Either variety 

 of refractoriness can be accounted for in eitlier of two ways. The 

 "inhibitory synapse" may be of such a kind as to produce a sub- 

 stance whicii raises the tlireshold of the neuron, or it may be so 

 placed that the local chsturbance prockiced by its excitation 

 opposes the alteration induced by tlie otlierwise excitatory syn- 

 apses. Inasmuch as position is already known to have such effects 

 in the case of electrical stimulation, the first hypothesis is to be 

 excluded unless and until it be substantiated, for the second 

 involves no new hypothesis. We have, then, two explanations of 

 inhibition based on the same general premises, differing only in 

 the assumed nervous nets and, consecjuently, in the time required 

 for inhibition. Hereafter we shall refer to such nerv'ous nets as 

 equivalent in the extended sense. Since we are concerned with properties 

 of nets which are invariant under equivalence, we may make the 

 physical assumptions which are most convenient for the calculus. 

 Many years ago one of us, by considerations impertinent to 

 this argument, was led to conceive of the I'esponse of any neuron 

 as factually equivalent to a proposition which proposed its ade- 

 quate stimulus. He therefore attempted to record the behavior of 

 complicated nets in the notation of the symbolic logic of proposi- 

 tions. The "all-or-none" law of nervous activity is sufficient to 

 insure that the activity of any neuron may be represented as a 

 proposition. Physiological relations existing among nervous activ- 

 ities correspond, of course, to relations among the propositions; 

 and the utility of the representation depends upon the identity 

 of these relations with those of the logic of propositions. To each 

 reaction of any neuron there is a corresponding assertion of a 

 simple proposition. This, in turn, implies either some other simple 

 proposition or the disjunction or the conjunction, with or without 

 negation, of similar propositions, according to the configuration 

 of the synapses upon and the threshold of the neuron in question. 

 Two difficulties appeared. The first concerns facilitation and ex- 

 tinction, in which antecedent activity temporarily alters responsive- 

 ness to subsequent stimulation of one and the same part of the 

 net. The second concerns learning, in which activities concurrent 

 at some previous time have altered the net permanently, so that 

 a stimulus which would previously have been inadequate is now 



