Did he? Some historians say that he suffered from bouts of insanity or depression. It would seem as though Cowper knew empirically, the subject of which he so often wrote.

The Chestnut Mare
All that is gold does not glitter,
Not all those who wander are lost
The old that is strong does not wither,
Deep roots are not reached by frost.
- - - -JRR Tolkien "Lord of the Rings"