"He [the analyst] must turn his own unconscious like a receptive organ toward the transmitting unconscious of the patient. He must adjust himself to the patient as a telephone receiver is adjusted to the transmitting microphone. Just as the receiver converts back into sound waves the electric oscillations in the telephone line which were set up by the sound waves, so the doctor's unconscious is able, from the derivatives of the unconscious which are communicated to him, to reconstruct that unconscious, which has determined the patient's free associations."
(Freud: 'Recommendations to Physicians Practising Psychoanalysis.' 1912.)