For very many people it will be a hard nut to crack if they are told to believe that when men grow old they do not become weak or even feeble-minded, but more psychic and more spiritual. Only, when the body is worn out, we can no longer express the psycho-spiritual which we have cultivated, through the body. It is like the case of a pianist: he might become a better and better player, but if his piano is worn out we cannot perceive this. If you were only to know his capabilities as a pianist from his play, you will not be able to gather much if the piano is out of tune and has broken strings. So that Kant, when he was an old man and “feeble-minded” was not weak minded as regards the spiritual world; there he had become glorious.
Source: Rudolf Steiner – GA 181 – Anthroposophical Life Gifts – Lecture III: Thoughts about the Life Between Death and Rebirth – Berlin, 2nd April 1918