Husserl suggested a method called free imaginative variation: When you feel you have a description of the essential characteristics of a category of phenomena, ask yourself, "What can I change or leave out without losing the phenomenon? If I color the triangle blue, or construct it out of Brazilian rosewood, do I still have a triangle? If I leave out an angle, or curve the sides, do I still have a triangle?" This may seem trivial and easy, but now try it regarding "being human:" Is a corpse human? A disembodied spirit? A person in a permanent coma? A porpoise with intelligence and personality? A just-fertilized egg? A six-month old fetus?
http://webspace.ship.edu/cgboer/phenandexist.html