Shaken from what?

Ought you not first to have acquired something by the use of reason, and then to have made that secure? But you are studying to be able to prove things in argument. Prove what, though? You are studying so as not to be shaken by fallacious arguments. Shaken from what? Show me first what you are watching over, what you are measuring, or what you are weighing; and then, accordingly, show me your balance.

~ Epictetus