between the primary and secondary indifference-states, and this
difference itself we define as the "engram" due to the stimulus. "Mnemic
phenomena" are defined as those due to engrams; in animals, they are
specially associated with the nervous system, but not exclusively, even
in man.

When two stimuli occur together, one of them, occurring afterwards,
may call out the reaction for the other also. We call this an "ekphoric
influence," and stimuli having this character are called "ekphoric
stimuli." In such a case we call the engrams of the two stimuli
"associated." All simultaneously generated engrams are associated; there
is also association of successively aroused engrams, though this is
reducible to simultaneous association. In fact, it is not an isolated
stimulus that leaves an engram, but the totality of the stimuli at any
moment; consequently any portion of this totality tends, if it recurs,
to arouse the whole reaction which was aroused before. Semon holds that
engrams can be inherited, and that an animal's innate habits may be due
to the experience of its ancestors; on this subject he refers to Samuel
Butler.

Semon formulates two "mnemic principles." The first, or "Law of
Engraphy," is as follows: "All simultaneous excitements in an organism
form a connected simultaneous excitement-complex, which as such works
engraphically, i.e. leaves behind a connected engram-complex, which
in so far forms a whole" ("Die mnemischen Empfindungen," p. 146).
The second mnemic principle, or "Law of Ekphory," is as follows:
"The partial return of the energetic situation which formerly worked
engraphically operates ekphorically on a simultaneous engram-complex"
(ib., p. 173). These two laws together represent in part a hypothesis
(the engram), and in part an observable fact. The observable fact is
that, when a certain complex of stimuli has originally caused a certain
complex of reactions, the recurrence of part of the stimuli tends to
cause the recurrence of the whole of the reactions.

Semon's applications of his fundamental ideas in various directions are
interesting and ingenious. Some of them will concern us later, but for
the present it is the fundamental character of mnemic phenomena that is
in question.

Concerning the nature of an engram, Semon confesses that at present it
is impossible to say more than that it must consist in some material
alteration in the body of the organism ("Die mnemischen Empfindungen,"
p. 376