The quote about is from Sigmund Freud, one of the founders of modern psychoanalytic thinking. What he means is that if a person keenly observes another he will gain some understanding that a person is disturbed, anxious or uneasy about something.
Observation is as important in psychotherapy as compassion or empathy and it is these two things together that make psychotherapy a powerful force for change.
The theory of personality that I studied in my professional training and that I most commonly refer to in my client work is Transactional Analysis. Founded by Eric Berne amongst others in the 1950’s and 60’s, the theories and models in Transactional Analysis are based on real world observable phenomena. They are therefore verifiable in the real world and visible in the client, through observation.
A good therapist seeks to provide a relationship which their client can use to grow and understand more about themselves. That relationship develops through a therapist enquiring about and observing their client in order to build and, very importantly, to check the therapist’s understanding of how things are both for and in their client.
Freud’s quote above seems confrontational, even angry. Therapy now is about developing a non judgemental understanding and through that shared understanding opportunities for growth and change, both in the client and the therapist.