Many questions one could ask about facial expressions seem to be fixed by the use of the very word expression. Facial expression, first, refers to facial behavior that suggests emotional meaning to an outside observer. Second, the term carries the implication that that facial behavior has the function or purpose of conveying such meaning. Third, it suggests that there exists something (say, an inner feeling) independently of that behavior to which the behavior called expression is added as an extra.

These aspects are not necessarily all true of the same behaviors. Facial behaviors may suggest emotional meanings to observers, but that may not be their function or purpose. Receiving the epithet “expressive” in fact says nothing about the nature of the behavior concerned. “Hasty” or “greedy” behaviors, for instance, are made to arrive as fast as one can at the object of desire, and not to inform others about one's state of mind. Also, nonbehavior may on occasion be highly expressive, such as underacting in the theater and Jesus's remaining silent under accusation. And there are phenomena that are expressive by suggesting emotional meanings in which no inner feelings of whatever produced the phenomena are involved, such as joyful bird songs, angry bursts of wind, sad music, nervous lines, and solemn penguins.

Most past and current theorizing on facial expression starts from the assumption that it expresses emotional feelings and exists for the sake of doing so. The study of expression pretty much originated in the philosophical problem of the knowledge of other minds.