What was their answer?
Rashi: No one can fulfill your request. No king ever requested something like this! Malbim ? if you knew a hidden matter, it would be hard for someone else to know it, and all the more so now that you forgot it!
Why did they say "there is no man on the land?"?
Malbim: The Kasdim believed that there are gods that dwell in the air, close to people. They know people's thoughts, and inform them to people who cling to them. (Other gods are totally separated from people; they dwell in Shamayim.) Those who cling to them can know others' thoughts ? but Chartumim, Ashafim and Kasdim do not deal with gods, only with natural science and astrology. No man on the land can know this, only gods who dwell in the air above the land. This is why no king ever requested a matter like this!
