Tickles are frequently cited as a pure example of sensation, entirely identified by the qualia they provide, and lacking any content or burden of information about the world. However the matter is more complex than that, as is evident from the fact that one cannot tickle oneself. The psychologist Havelock Ellis (1859–1939) thought of the reaction as a defence against premature sexual advances, but this does not explain why the soles of the feet are ticklish. One account of the emotional essence of being tickled is ‘the swift interaction of moments of nascent fear with a joyous recognition of harmlessness.’ Aristotle refers mysteriously to ‘those who do not get tickled themselves if they tickle someone else first’ (Nicomachean Ethics, 1150 b 22). See also laughter.