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A horse walks into a bar. The bartender asks the horse if it's an alcoholic considering all the bars he frequents, to which the horse replies "I don't think I am." POOF! The horse disappears.
This is the point in time when all the philosophy students in the audience begin to giggle, as they are familiar with the philosophical proposition of Cogito ergo sum, or I think, therefore, I am.
But to explain the concept beforehand would be putting Descartes before the horse.
@Bobbelieu
Réné Descartes is sitting in a bar and finishes off the glass of beer he's drinking.
"Have another?" asks the bartender.
"I think not, says Descartes, and VANISHES.