The following excerpt has more to do with defining obscenity than pornography, but...
In 1964, Justice Potter Stewart tried to explain "hard-core" pornography, or what is obscene, by saying, "I shall not today attempt further to define the kinds of material I understand to be embraced . . . [b]ut I know it when I see it "
Excerpted from...
http://library.lp.findlaw.com/articl...tionallaw_1_86