DENVER — The spiral notebook is a road map to murder, filled with plans, diagrams and to-do lists that James E. Holmes laid out in scrupulous detail before carrying out a shooting rampage in a Colorado movie theater. In his own handwriting, he plans a “mass murder spree” and considers theaters and times to attack for “maximum casualties.” He also plots his own psyche with pages of self-diagnosis of what he calls his “broken mind.”

As his murder trial enters its second month, this notebook has become a Rorschach test of the thoughts of Mr. Holmes, the neuroscience graduate student who killed 12 people in one of the country’s worst mass shootings in recent years.

Prosecutors say that he was sane and methodical, planning his actions with murderous intent. Defense lawyers, who have entered his plea of not guilty by reason of insanity, say his writing brims with “a whole lot of crazy” — delusions about death, human worth and “negative infinity” that were the product of a profoundly diseased mind.

The 12 jurors will be forced to examine the hazy border between mental illness and legal insanity. Key to the case is whether Mr. Holmes, despite his ravings and struggles with mental illness, was able to distinguish the difference between right and wrong, and was legally responsible for his actions, when he opened fire on a midnight movie screening in Aurora, Colo., in July 2012.