WHEN does the deterioration of your brain rob you of your identity, and when does it not?

Alzheimer’s, the neurodegenerative disease that erodes old memories and the ability to form new ones, has a reputation as a ruthless plunderer of selfhood. People with the disease may no longer seem like themselves.

Neurodegenerative diseases that target the motor system, like amyotrophic lateral sclerosis, can lead to equally devastating consequences: difficulty moving, walking, speaking and eventually, swallowing and breathing. Yet they do not seem to threaten the fabric of selfhood in quite the same way.

Memory, it seems, is central to identity. And indeed, many philosophers and psychologists have supposed as much. This idea is intuitive enough, for what captures our personal trajectory through life better than the vault of our recollections?

But maybe this conventional wisdom is wrong. After all, the array of cognitive faculties affected by neurodegenerative diseases is vast: language, emotion, visual processing, personality, intelligence, moral behavior. Perhaps some of these play a role in securing a person’s identity.