There are other ways in which Browning's thought is especially mystical,
as, for instance, his belief in pre-existence, and his theory of
knowledge, for he, like Plato, believes in the light within the soul,
and holds that--

To know
Rather consists in opening out a way
Whence the imprisoned splendour may escape,
Than in effecting entry for a light
Supposed to be without.

_Paracelsus_, Act I.

But the one thought which is ever constant with him, and is peculiarly
helpful to the practical man, is his recognition of the value of
limitation in all our energies, and the stress he lays on the fact that
only by virtue of this limitation can we grow. We should be paralysed
else. It is Goethe's doctrine of _Entbehrung_, and it is vividly
portrayed in the epistle of Karshish. Paracelsus learns it, and makes it
clear to Festus at the end.

The natural result of Browning's theory of evil, and his sense of the
value of limitation, is that he should welcome for man the experience of
doubt, difficulty, temptation, pain; and this we find is the case.

Life is probation and the earth no goal
But starting point of man ...
To try man's foot, if it will creep or climb
'Mid obstacles in seeming, points that prove
Advantage for who vaults from low to high