and we may feel safe that their expressions are not conventional.


From the reasons above assigned, namely, the fleeting nature
of some expressions (the changes in the features being often
extremely slight); our sympathy being easily aroused when we
behold any strong emotion, and our attention thus distracted;
our imagination deceiving us, from knowing in a vague manner
what to expect, though certainly few of us know what the exact
changes in the countenance are; and lastly, even our long
familiarity with the subject,--from all these causes combined,
the observation of Expression is by no means easy, as many persons,
whom I have asked to observe certain points, have soon discovered.
Hence it is difficult to determine, with certainty,
what are the movements of the features and of the body,
which commonly characterize certain states of the mind.
Nevertheless, some of the doubts and difficulties have,
as I hope, been cleared away by the observation of infants,--
of the insane,--of the different races of man,--of works of art,--
and lastly, of the facial muscles under the action of galvanism,
as effected by Dr. Duchenne.

But there remains the much greater difficulty of understanding
the cause or origin of the several expressions, and of judging whether
any theoretical explanation is trustworthy. Besides, judging as
well as we can by our reason, without the aid of any rules,
which of two or more explanations is the most satisfactory, or are
quite unsatisfactory, I see only one way of testing our conclusions.
This is to observe whether the same principle by which one expression can,
as it appears, be explained, is applicable in other allied cases;