And as for sadness, how can it be profitable to holy charity, seeing that joy is ranked amongst the fruits of the Holy Ghost, coming next to charity? Still, the great apostle says: The sorrow that is according to God worketh penance unto salvation which is lasting: but the sorrow of the world worketh death.  There is then a sorrow or sadness according to God, which is employed either by sinners in penance, or by the good in compassion for the temporal miseries of their neighbours, or by the perfect in deploring, bemoaning and condoling the spiritual calamities of souls. For David, S. Peter, Magdalen, wept for their sins; Agar wept when she saw her son almost dead of thirst; Jeremias over the ruin of Jerusalem; Our Saviour over the Jews; and his great Apostle sighing says these words: Many walk of whom I have told you often (and now tell you weeping) that they are enemies of the cross of Christ. 
There is then also a sadness of this world, which likewise proceeds from three causes. For -- 1°. It comes sometimes from the infernal enemy, who by a thousand sad, melancholy and disturbing suggestions obscures the understanding, weakens the will, and troubles the whole soul: and as a thick mist fills the head and breast with rheum, and by this means makes respiration difficult, and greatly incommodes the traveller; so the evil spirit, filling man's mind with sad thoughts, deprives it of facility in aspiring to God, and possesses it with an extreme tedium and discouragement, in order to bring it to despair and perdition. They say there is a fish called the sea-toad, surnamed the sea-devil, which stirring and spreading the mud troubles the water round about it so as to hide itself therein as in an ambush, from whence, as soon as it perceives poor little fishes, it darts upon them, kills and devours them: whence perhaps has come the common expression -- fishing in troubled waters. Now it is the same with the devil of hell as with the devil of the sea; for he makes his ambush in sadness, and then, having troubled the soul with a multitude of sad thoughts cast hither and thither in the understanding, he makes a charge upon the affections, bearing them down with distrust, jealousies, aversions, envies, superfluous apprehensions of past sins, adding withal a number of vain, sour and melancholy subtleties of the imagination, that all reasons and consolations may be rejected.
2°. Sadness sometimes also proceeds from one's natural disposition, when the melancholy humour predominates in us: and this is not vicious in itself, yet our enemy makes great use of it to weave and prepare a thousand temptations in our souls. For as spiders scarcely ever spin their webs save when the weather is dull and the sky cloudy; so this malign spirit never finds as much facility in spreading the nets of his suggestions in sweet, kindly and bright souls, as he has with the gloomy, sad and melancholy; for these he easily disturbs with vexations, suspicions, hatreds, murmurings, censures, envies, sloth and spiritual numbness.
3°. Lastly, there is a sadness which the various accidents of life bring upon us. What manner of joy shall be to me, said Tobias, who sit in darkness, and see not the light of heaven  Thus was Jacob sad on the news of the death of his Joseph, and David for that of his Absalom. Now this sadness is common to the good and the bad; but to the good it is moderated by acquiescence in and resignation to the will of God: as we see in Tobias, who gave thanks to the Divine Majesty for all the adversities which came upon him, and in Job, who blessed the name of the Lord for them, and in Daniel, who turned his griefs into songs of joy. As to worldlings, on the contrary, this sadness is an ordinary thing with them, and spreads out into regrets, despair, and deadness of soul: for they are like apes and monkeys, which are always sullen, sad and peevish at the waning of the moon, as, on the contrary, at the new moon, they leap, dance and play their apish tricks. The worldling is out of temper, uncivil, bitter and gloomy when temporal prosperity fails him; and in abundance he is almost always boastful, foolishly elated and insolent.
Indeed the sadness of true penitence is not so much to be named sadness as displeasure, or the sense and detestation of evil; a sadness which is never troubled nor vexed; a sadness which does not dull the spirit, but makes it active, ready and diligent; a sadness which does not weigh the heart down, but raises it by prayer and hope, and causes in it the movements of the fervour of devotion; a sadness which in the heaviest of its bitternesses ever produces the sweetness of an incomparable consolation, according to the precept of the great S. Augustine: "Let the penitent sorrow always, yet always rejoice for his sorrow." "The sadness," says Cassian, "which works solid penitence, and that desirable repentance of which one never repents, is obedient, affable, humble, mild, sweet, patient, -- as being a child and scion of charity: so that spreading over every pain of body and contrition of spirit, and being in a certain way joyous, courageous, and strengthened by the hope of doing better, it retains all the sweetness of gentleness and longanimity, having in itself the Fruits of the Holy Spirit, which the holy Apostle recounts: Now the Fruits of the Spirit are charity, joy, peace, longanimity, goodness, benignity, faith, mildness, continency." Such is true penitence, and such is right sadness, which in good sooth is not really sad or melancholy, but only attentive and earnest to detest, reject and hinder the evil of sin for past and for future. And indeed we often see repentances which are very eager, troubled, impatient, wet-eyed, bitter, given to groans, very crabbed and melancholy, which at last turn out fruitless and lack all true amendment, because they do not proceed from the true motives of the virtue of penitence, but from selfish and natural love.
The sorrow of the world worketh death,  says the Apostle; we must, therefore, Theotimus, carefully avoid and banish it as much as we can. If it be from nature, we must repulse it by contradicting its movements, turning it aside by the practices suitable to that purpose, and using the remedies and way of life which physicians themselves may judge best. If it come from temptation, we must clearly open our mind to our spiritual father, who, will prescribe for us the method of overcoming it, according as we have said in Part IV. of the Introduction to the Devout Life. If it arise from circumstances, we will have recourse to the teaching of Book VIII., in order to see how grateful tribulations are to the children of God, and how the greatness of our hopes for eternal life ought to make all the passing events of the temporal almost unworthy of thinking about.
At last, in all the sadness which may come upon us, we must employ the authority of the superior will to do all that should be done in favour of divine love. There are indeed actions which so depend upon the corporal disposition and constitution that we have not the power to do them just as we please: for the melancholy-disposed cannot keep their eyes, or their words, or their faces, in the same good grace and sweetness as they would do if they were relieved from this bad humour; but they are quite able, though without this good grace, to say gracious, kind, and civil words, and, in spite of inclination, to do what reason requires as to words and works of charity, gentleness and condescension. We may be excused for not being always bright, for one is not master of cheerfulness to have it when one will; but we are not excusable for not being always gracious, yielding and considerate; for this is always in the power of our will, and we have only to determine to keep down the contrary humour and inclination.