The notion of differential diagnosis constitutes today an original subject of study in clinical psychopathology: whereas psychiatric classifications stay essentially attached to symptomatic behaviors whose grouping remains unpredictable, even secondary, the research in psychoanalytic psychopathology occupies an essential place in this respect, and it demonstrates more and more subtle and pertinent possibilities thanks to the refinement of the investigation techniques. The Rorschach, considered as an identity test and a test of boundaries, allows to put to the test the solidity of individuation processes and the constitution of adequate borders between inside and outside: sensitive on the one hand, to the disintegration of self-representation; on the other hand, it turns out to be just as sensitive when it comes to look for signs of construction, to search for structuring points of reference and to rely on containing outlines. The TAT offers more appeal to relations which mobilize a work of thought in building a story around an imaginary scenario: so it involves making use of a linking activity especially in the arrangement of instinctual conflicts, particularly within the oedipal conflict; but we also know about the effects of this material on the reactivation of depressive and narcissistic problematics. Articulation of these essential problematics is analyzed in its unique configurations, which allows to comprehend different modes of organization of psychical functioning thanks to joint study of the Rorschach and of the TAT. To illustrate this subject, the author presents a specific psychopathological situation, putting the problem of differential diagnostic between narcissistic personality and obsessional neurosis.