you derive your sense of self from the content and activity of your mind. Because you believe that you would cease to be if you stopped thinking. As you grow up, you form a mental image of who you are, based on your personal and cultural conditioning. We may call this phantom self the ego. It consists of mind activity and can only be kept going through constant thinking. The term ego means different things to different people, but when I use it here it means a false self, created by unconscious identification with the mind.
