Hello again—my second greeting today. In the main episode we heard of Gage, whose brain injury changed his personality until acquaintances felt he was no longer the same person.This shows that stable personality depends on a healthy, well-functioning brain.

Gage suffered an accident, but diseases also alter brain function. Do they alter personality too?

Indeed. For example,Alzheimer's disease,a severe degenerative brain disorder, is familiar through early symptoms such asbehavioral changeandmemory loss.As it progresses, personality can change drastically and eventually disappear entirely.Strokealso damages parts of the brain and may strongly affect personality. Somebrain surgeriescan alter its biological foundations, making a once deeply familiar person seem strange to loved ones (Grunberg, Klein, & Brown, 1998).

These phenomena challenge simplemind–body dualismand raise an ethical question:What within us defines us, and can it be perceived to constitute a person's sense of personal identity? Put another way:Does something exist whose loss removes identity so thoroughly that ‘I’ no longer applies? Memory, personality, emotion, intelligence, morality, or something else—which matters most in defining you?

Many philosophers considermemorymost important to identity. If everything precious from one's past is forgotten, how can one know who one is?Others can reconstruct the past verbally, but without its experience and emotion, hearing one's own story resembles hearing a stranger's—empty and disorienting.

Yet memory is not everything. People with accident-induced amnesia can assess whether they are introverted, careful, or friendly, agreeing closely with relatives and friends. They may not explain future goals or why they became this way, but with normal brain function they soon rebuild these through new life. This suggestspersonality is most important to personal identity, while memory helps confirm that I remain I.

From an observer's view, we also use factors to decide whether someone remains the person we knew. Likewise,what makes a person themselves to you, and what change would make them no longer that person?