Mentally

mentally capable

mentally ill

This is a problem that needs to be solved mentally, not with brute force.

The novel he lends his name to is an account of that life, taking more or less the form of a memoir set down in the years immediately after World War II by an old man mentally preparing for death, not quite at home in the twentieth century (and a little proud of it, stubbornly clinging to his eartrumpet), whose significant memories reach back to the early 1890s and beyond.