Does it concern anyone else how squishy your mind architecture is? You can be unknowably changed without any warning. You could develop a tumor and have different parts of your brain seriously affected. You could be poisoned by mercury or infected by a prion. Even a normal virus could make its way into your brain through some weird means (the current hypothesis for Alzheimer's). An accident could cause physical trauma to the brain. All these things could change a number of things about you. Maybe it's a slight behavioral change or maybe a large one. Maybe it only affects the motor abilities of your body and leads to you uncontrollably shitting yourself. Or maybe you become a vegetable for the rest of your life. Or maybe you're just intellectually impaired so you find it harder to think about things even though you have the same processes as before, even though you remember when you used to not be retarded.