Through Alzheimer’s, of course. Through alcohol killing your brain cells, of course. I can only hope that medical science will advance so far, that we’ll have microscopic robots in our bodies repairing our brain & other cells so that we don’t lose our intelligences anymore. (I’m talking about robots tiny enough that 1,000 of them clumped together would combine to the size of one individual red blood cell. At least 1,000.)