However we select from nature a complex [of phenomena] using the criterion of simplicity, in no case will its theoretical treatment turn out to be forever appropriate (sufficient).... I do not doubt that the day will come when [general relativity], too, will have to yield to another one, for reasons which at present we do not yet surmise. I believe that this process of deepening theory has no limits.
Albert EinsteinAs our circle of knowledge expands, so does the circumference of darkness surrounding it.
Albert EinsteinThe daily struggle does not arise from a purpose or a program, but from an immediate need.
Albert Einstein