At the same time, clear knowledge that evil is illusion would defeat its own end and paralyse all moral effort, for evil only exists for the development of good in us.
Type needs antitype: As night needs day, as shine needs shade, so good Needs evil: how were pity understood Unless by pain?
This is one reason why Browning never shrank from the evil in the world, why indeed he expended so much of his mind and art on the analysis and dissection of every kind of evil, laying bare for us the working of the mind of the criminal, the hypocrite, the weakling, and the cynic; because he held that--
Only by looking low, ere looking high Comes penetration of the mystery.
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.
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 And makes the stumbling-block a stepping-stone.
_The Ring and the Book_: The Pope, 1436-7, 410-13.
It is this trust in unending progress, based on the consciousness of present failure, which is peculiarly inspiriting in Browning's thought, and it is essentially mystical. Instead of shrinking from pain, the mystic prays for it, for, properly met, it means growth.