Convex and concave: conceptual boundaries in psychology, now and then (but mainly then).

My title is derived from G. H. Lewes's psychological magnum opus Problems of Life and Mind (1874–79). Lewes's image is a metaphor for the relation of mind to brain, or more generally of the mind to the nervous system: “every mental phenomenon has its corresponding neural phenomenon (the two being as convex and concave surfaces of the same sphere, distinguishable yet identical)” (Problems: First Series 1: 112). His point is that, though the two entities can be analytically distinguished, they are as necessarily linked as the two surfaces of a bending plane. Like the recto and verso of a sheet of paper, or signifier and signified in the linguistic sign, one can make an interpretative separation of the two, but not an ontological one. It is a characteristically deft metaphor by Lewes to express a notoriously vexed relationship, not only in Victorian psychology but also in modern thinking today.