What was Lincoln Rhyme's greatest enemy after his accident?

While his physical condition is precarious, much of the anguish Lincoln feels is mental: What had kept Lincoln Rhyme sane since his accident—what had kept him from finding some Jack Kevorkian to help with assisted suicide—were mental challenges like this.... Lincoln Rhyme's greatest enemy wasn't the spasms, phantom pain or dysreflexia that plague spinal cord patients; it was boredom.