This was a fairly common occurrence in the schools: a question would be stated, and a respondent would take up a position on the question and hold it against the arguments of the learned people present. What made Pico's unusual was that he proposed to hold his own against all comers on no fewer than nine hundred separate points of philosophy.
It never happened, because the Pope got hold of the proposed list of points and declared that several of them were not lawful for Christians to discuss. However, Pico published an apologia about it-- explanation and defense of the idea after the fact, but also a genuine apology-- in which he reprinted the speech he had planned to give at the beginning of the disputation. This speech, the Oration on the Dignity of Man, is now the work for which he is principally remembered.
It's one of the core documents of humanism, because it proposes the idea that humanity has dignity and worth not necessarily because of the place we occupy in the order of the cosmos, but because human beings can make themselves into whatever they desire to be and can be so many infinitely varied things. An animal, says Pico, comes into the world with whatever it is going to be imprinted upon it, but a human being may be noble or evil, intellectual or sensual, brave or timorous; certainly we have some inclination towards one thing or another, but there is a substantial element of decision and will.
This idea, which was original to Pico, is very well-stated and argued, though one can see immediately in the oration why it led him into things that the authorities disapproved of, since he states that there is therefore no reason that a person striving after the good should not attain a level coequal to that of the angels-- and then goes into careful discussion about which personal qualities one would have to cultivate to be like each sort of...