FOUNDATION AND EMPIRE by ISAAC ASIMOV

How did you get it out of them?
How do you know they're telling the truth?"

Forell frowned. "I wasn't gentle, good sir.
I knocked them about, drugged them crazy,
and used the Probe unmercifully.
They talked. You can believe them."

"In the old days," said the third man, with sudden irrelevance,
"they would have used pure psychology.
Painless, you know, but very sure. No chance of deceit."