A being that is both omnipotent and omniscent. This means that it can both do anything and see everything. Obviously, since it can see everything, it can see into the
future. If it can see into the
future, it can choose its path, because it is omnipotent. This means it has seen the
wrong future, or is unable to change the future, thus contradicting itself. Or, as
said by an ancient Greek philosopher,
Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil? Is he neither able nor willing? Then why call him God? ~Epicurus