Another point of resemblance with Eckhart is suggested by his words:  “That foolish people take evil for good, and good for evil.”  Browning’s theory of evil is part of the working-out of his principle of what may be called the coincidence of extreme opposites.  This is, of course, part of his main belief in unity, but it is an interesting development of it.  This theory is marked all through his writings; and, although philosophers have dealt with it, he is perhaps the one poet who faces the problem, and expresses himself on the point with entire conviction.  His view is that good and evil are purely relative terms (see The Bean-stripe), and that one cannot exist without the other.  It is evil which alone makes possible some of the divinest qualities in man—­compassion, pity, forgiveness patience.  We have seen that Shelley shares this view, “for none knew good from evil”; and Blake expresses himself very strongly about it, and complains that Plato “knew nothing but the virtues and vices, the good and evil....  There is nothing in all that....  Everything is good in God’s eyes.”  Mysticism is always a reconcilement of opposites; and this, as we have seen in connection with science and religion, knowledge and love, is a dominant note of Browning’s philosophy.  He brings it out most startlingly perhaps in The Statue and the Bust, where he shows that in his very capacity for vice, a man proves his capacity for virtue, and that a failure of energy in the one implies a corresponding failure of energy in the other.

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.

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.