And therefore I must explain the origin of this pain, that is to say, the cause that occasions this grief in the mind, as if it were a sickness of the body.

For as physicians think they have found out the cure when they have discovered the cause of the distemper, so we shall discover the method of curing melancholy when the cause of it is found out.
XI.

The whole cause, then, is in opinion; and this observation applies not to this grief alone, but to every other disorder of the mind, which are of four sorts, but consisting of many parts.

For as every disorder or perturbation is a motion of the mind, either devoid of reason, or in despite of reason, or in disobedience to reason, and as that motion is excited by an opinion of either good or evil; these four perturbations are divided equally into two parts: for two of them proceed from an opinion of good, one of which is an exulting pleasure, that is to say, a joy elated beyond measure, arising from an opinion of some present great good; the other is a desire which may fairly be called even a lust, and is an immoderate inclination after some conceived great good without any obedience to reason.

Therefore these two kinds, the exulting pleasure and the lust, have their rise from an opinion of good, as the other two, fear and grief, have from an opinion of evil.
For fear is an opinion of some great evil impending over us, and grief is an opinion of some great evil present; and, indeed, it is a freshly conceived opinion of an evil so great that to grieve at it seems right: it is of that kind that he who is uneasy at it thinks he has good reason to be so.