Humanity is that Creature that Laughs

"They divide property into four: what is an accident of a certain species alone, even if not of it all (as doctoring or doing geometry [is said] of man); what is an accident of all the species, even if not of it alone (as being biped of man); what holds of it alone and of all of it and at some time (as growing grey in old age of man); and fourthly, where 'alone and all and always' coincide (as laughing (to gelastikon) of man). For even if man does not always laugh, he is said to be laughing not in that he always laughs but in that he is of such a nature (pephukenai) as to laugh--and this holds of him always, being conatural (sumphuton), like neighing (to chremetistikon) of horses.  And they say that these are properties in the strict sense, because they convert: if human, laughing; and if laughing, human."

--Porphyry, Eisagoge 12.13-21