The difference between kinds of smile (e.g., a cruel smile, ironic smile, joyful smile) may be no more than a minute difference in the orientation of the facial features. The difference may reside in no more than a thousandth of an inch, but we recognize the difference, and not by measuring it. Moreover, we do not explain our different descriptions by reference to measurement.
We cringe when we hear of another’s fear, and grit our teeth when confronted with someone’s anger (motor mimicry). And we can reproduce such expressions without looking in the mirror, that is to say, without checking on the exact orientation of our features. It is a fact that human beings are extremely sensitive to the play of features on a human face. ‘We see emotion’ (RPP II 570).
A small quantitative difference can make a significant qualitative difference. I think here of Moebius Syndrome. People with Moebius are born without the 6th and 7th cranial nerves which innervate the facial muscles, and so they cannot make facial expressions. This leads to difficulties in communicating emotion on the face, and the ability to read it in others, altered understanding of character and selfhood, being less available to others, and a disturbed self-other balance.
Just as pain can get no foothold in the absence of behavior that expresses pain, the same applies to the emotional experiences of others which we observe in human behavior. Wittgenstein makes out a strong case for thinking that the intelligibility of psychological terms presupposes the possibility of behavioural manifestations. If I cannot make facial expressions, I am denied a mental life by almost every other person except those who know me exceptionally well, and I become less available to others, and therefore less available to myself. The transition ‘from quantity to quality’.