When one considers the functioning of the mind it is impossible to represent it like the functioning of the heart or the kidney because the end products are different. The thoughts and the feelings are not measurable like the blood pressure, the pulse or the urine. Then we need another model than the medical model to understand the functioning of the mind. Freud conceptualized his metapsychology in response to the impossibility to represent the functioning of the mind according to the medical model. The followers of Freud kept this metapsychology and brought important contributions, especially in the field of topology.