There is usually more than one theoretical view of a hypercomplex situation, which may be cognitively complementary, but they bring about different results when used to orient our practice, on account of their underlying assumptions, values, and intentions. The authors compare two approaches to work with groups: Foulkes’s group analysis and Kaës’s psychoanalytic approach to groups, exploring their coincidences and differences, through their respective concepts of the matrix and the group psychic apparatus. Psychoanalysis starts from the assumption of an isolated individual subject, and then constructs the additional dimension of relations and collective life. Group analysis takes as its starting point the assumption of the primary and essential relational and social nature of the human being, and the subject is a secondary construction that emerges from the initial participatory existence. The concept of the internal group in Kaës is strictly intrapsychic and abstract, while in Pichon-Rivière it is experiential and introjective. Psychoanalysis and group analysis are based on two different conceptions of human existence, and this is clearly shown by the authors’ clinical vignette. Studies like this contribute to a better understanding among the various traditions and schools of analytic work with groups.