La diagonale du fou

The use of the diagonal is one of the processes with which geometry helps the conceptualisation and realisation of a masterpiece. Diagonals also contribute to highlight pieces of scientific evidence, while at the same time they demonstrate the impact of the unconscious on painters' artistic creativity. The article reviews images showing diagonals in a rectangle that demonstrate a need for geometry. This need is independent of mathematical reasoning, however they are full of symbolsthat express obsessive fears and psychotic anxiety.