'Bunter,' said Lord Peter, as the kitchen door closed behind them, 'do you know why I am doubtful about the success of those rat experiments?'
'Meaning Dr Hartman's, my lord?'
'Yes. Dr Hartman has a theory. In any investigation, my Bunter, it is most damnably dangerous to have a theory.'
'I have heard you say so, my lord.'
'Confound you - you know it as well as I do! What is wrong with the doctor's theories, Bunter?'
'You wish me to reply, my lord, that he only sees the facts which fit into the theory.'
'Thought-reader!' exclaimed Lord Peter bitterly.
From "The Footsteps That Ran," in Dorothy L. Sayers's Lord Peter Views the Body (Coronet Books: 1992) p. 158.