Is it not a great contradiction to talk of conceiving a thing which is unconceived?

One of Berkeley's arguments for the claim that esse is percipi is his famous Master Argument. This argument seeks to establish that it is impossible to conceive of an unconceived object, because the moment you try to conceive of the object, it is conceived (by you). This argument was wildly influential in the history of philosophy, inspiring such thinkers as Kant, Hegel, and Russel.