You pose a question that has vexed philosophers for thousands of years. How can we know the answer? Is it even knowable? How do we know if it's knowable? Do I have enough napkins to last the weekend? The mind reels....
Absolutely sawarbrules - I believe that the association with horses is much older than the urban definition meaning "I'm worn out" (presumably meaning I'm so tired I am only fit for the knacker's yard?)