Sadness is of two distinct types. The first type is the one that has a clearly identifiable cause such as the death of a beloved relative or the loss of wealth or something that the person greatly values. The second type has no obvious cause. It is a sudden distress and gloom that descends over the affected person preventing him, most of the time, from the exuberance of activity and the enjoyment of the usual pleasures of this world. The person afflicted is generally unaware of any clear reason for his dejection.
The causes behind the latter type of sadness or depression for which there is no known reason are related to bodily symptoms such as the impurity of the blood, its coolness and the changes in its contents. The treatment for this symptom is physical and psychological. As for the physical, it concentrates on purifying the blood, increasing its temperature and making it lighter. The psychological is limited to gentle encouraging talk that brings back some happiness as well as listening to music and songs and similar activities that emotionally give warmth to the gloomy.
One of the thought mechanisms to treat the sadness or depression that has a known reason i.e. loss of a loved relative or inability to obtain something one desperately wants is to weigh up the excessive bodily harm that continued sadness and depression can cause to one's body with the urge to mourn over his loss. Logical thinking would convince the person in question that his bodily health should be the most beloved thing to him. He should not accept to trade it in for any sum of money or relatives. The fact that a person feels sad and depressed for presumed loss is actually because he loves his body and soul and wants to please himself with what he failed to obtain or to stop the loss from happening. Destroying his health in agony over what has been lost, would be akin to someone selling out his capital to gain some little profit.
Another maneuver is for one to understand and realize that life in this world, by its very nature, is not the abode of perpetual joy and happiness, nor the abode of avoiding any loss of loved ones or sought after desires. One should look around to see if anyone has been spared such losses and bereavements. None will be found. If this is the way of things then one should deeply convince oneself that all the pleasures one obtains in life are but an additional gift that should be enjoyed with delight and that the losses one suffers and (those things) which one is unable to attain should not cause one much sorrow and bereavement.
A further mental approach is to realize that any hardship or damaging loss that besets one is similar or even less severe than the predicaments other people have suffered or are now suffering from. It is one of the characteristics of human nature to find solace in one's hardship when one discovers that it is shared by many other people. Furthermore, one should always remember that incidents that cause people to feel sad or to grieve are part of the engraved nature of this life.
Finally, by surveying one's own as well as other's experiences, one will come to the realization that all incidents of sorrow and grief are destined to be forgotten and that with the passing of days the agony would certainly diminish. One must take cognizance of the fact that the most saddening moment of an incident is its inception and that the days that come after will certainly reduce its painful effects until it is gradually pushed into forgetfulness. This kind of mental maneuver is bound to bring about a quick feeling of comfort or even happiness and pleasure.
"Abu Zayd al-Balkhi's Sustenance of the Soul: the Cognitive Behavior Therapy of a Ninth Century Physician" - Malik Badri, pp. 48-52