This dissertation describes the building of knowledge about dementia in two renowned clinics in the world of neuroscience: one situated in America, the other in France. How do these teams of neuroscientists distinguish the demented person from the reasonable person?

Drawing on 18 months of ethnographic research, I analyze how the understanding of frontotemporal dementia (FTD), a neurodegenerative disease that is defined in opposition to Alzheimer's disease (AD), allows neuroscience to explore, from the destruction of our brain, our emotional and social being. Well equipped with contemporary and American cerebral theories on "empathy" and "sociality", I then examine how teams of neuroscientists diagnose the pathology of the social and emotional being in an individual patient. How do they reach a diagnosis in a context at times marked by uncertainty? I lay out these uncertainties that point to the weakness of an exclusive positive definition of knowledge and I study how medical expertise can be conceived as connoisseurship, bringing to light the "feelings" and "tastes" of these doctors for the disease as a decisive tool for the making of a diagnosis. I conclude on the differences I observed in the understanding of FTD in the French clinic versus in the American clinic.