). It is, in fact, hypothetical, invoked for theoretical uses, and
not an outcome of direct observation. No doubt physiology, especially
the disturbances of memory through lesions in the brain, affords grounds
for this hypothesis; nevertheless it does remain a hypothesis, the
validity of which will be discussed at the end of this lecture.

I am inclined to think that, in the present state of physiology, the
introduction of the engram does not serve to simplify the account of
mnemic phenomena. We can, I think, formulate the known laws of such
phenomena in terms, wholly, of observable facts, by recognizing
provisionally what we may call "mnemic causation." By this I mean that
kind of causation of which I spoke at the beginning of this lecture,
that kind, namely, in which the proximate cause consists not merely of a
present event, but of this together with a past event. I do not wish to
urge that this form of causation is ultimate, but that, in the present
state of our knowledge, it affords a simplification, and enables us
to state laws of behaviour in less hypothetical terms than we should
otherwise have to employ.

The clearest instance of what I mean is recollection of a past event.
What we observe is that certain present stimuli lead us to recollect
certain occurrences, but that at times when we are not recollecting
them, there is nothing discoverable in our minds that could be called
memory of them. Memories, as mental facts, arise from time to time,
but do not, so far as we can see, exist in any shape while they are
"latent." In fact, when we say that they are "latent," we mean merely
that they will exist under certain circumstances. If, then, there is
to be some standing difference between the person who can remember a
certain fact and the person who cannot, that standing difference must
be, not in anything mental, but in the brain. It is quite probable that
there is such a difference in the brain, but its nature is unknown and
it remains hypothetical. Everything that has, so far, been made matter
of observation as regards this question can be put together in the
statement: When a certain complex of sensations has occurred to a man,
the recurrence of part of the complex tends to arouse the recollection
of the whole. In like manner, we can collect all mnemic phenomena in
living organisms under a single law, which contains what is hitherto
verifiable in Semon's two laws. This single law is:

IF A COMPLEX STIMULUS A