since we are creatures of schema it's difficult for us to know or define without an opposite. rather than one greatest creation i throw out the following.
1. death (without death life has no meaning and would turn into one long procrastination of doing anything)
2. self-awareness (otherwise this conversation is mute)
3. pussy (might be #1 depends on how long it's been since the last... my friends and i number one saying in college was "pussy is god." so maybe the question should be "what is pussy's greatest creation?" which would be answered: bliss
