Imagination is a good servant, and a bad master.
 Agatha Christie, The Mysterious Affair at Styles (1920). copy citation

Context

“Events proved that I was right in my supposition. ” “ That was very ingenious, ” “ I must confess that the conclusions I drew from those few scribbled words were quite erroneous. ”
“ You gave too much rein to your imagination. Imagination is a good servant, and a bad master. The simplest explanation is always the most likely. ” “ Another point—how did you know that the key of the despatch-case had been lost? ” “ I did not know it. It was a guess that turned out to be correct.” source