Term introduced by John Cottingham for an alternative to the usual interpretation of Rene Descartes (1596-1650) as a dualist of mind of body for whom all phenomena involving thought or consciousness belong to mind and all those involving extension belong to body.
The trialist interpretation keeps the two substances of mind and body, but introduces a third attribute, sensation, alongside thought and extension and belonging to the union of mind and body.
Among other things this allows animals, which do not have thought, to be regarded as having sensation and not, as in the traditional dualist interpretation, as being mere automata. Cottingham does not claim, however, that Descartes developed this trialism with complete consistency.
J Cottingham, 'Cartesian Trialism', Mind (1985)