Such a want-wit sadness makes of me

In sooth I know not why I am so sad,
It wearies me, you say it wearies you;
But how I caught it, found it, or came by it,
What stuff 'tis made of, whereof it is born,
I am to learn:
And such a want-wit sadness makes of me,
That I have much ado to know myself...  

                     The Merchant of Venice  
                     Act 1, Scene 1 

Shakespeare knew.

These are the opening lines of The Merchant of Venice, in which that very merchant, Antonio, describes his condition to his friend, Salerio. It is a perceptive description of a certain kind of depression, with which you feel a thing like sadness but which isn't actual sadness - you have nothing in particular to be sad about. (Although this condition often causes you to mine thoughts and memories that increase your sadness, it tempts you to inflate things or deflate things as necessary, to see the bad as worse and the worst in everything.) As Antonio says, the feeling wearies you; you are sure it wears on your friends and family, your co-workers and your neighbors. Certainly it wears you down physically and emotionally, but you cannot trace its origins in any very satisfactory way. Whatever it is and wherever it came from, it clogs and slows your mind, robs you of whatever spirit and wit you might formerly have possessed, and just makes you feel generally dumber than you used to feel. 

Shakespeare being Shakespeare, surely he was capable of describing this condition perfectly without ever having experienced it. However, it's what ensues that makes me think the author may well have suffered himself from depression. As the scene progresses, Antonio's friends try to cheer him up, despite his suggestions to them that it's futile, that "I hold the world...a stage, where every man must play a part,/ and mine a sad one." Still, they keep trying. "Let me play the fool," says Gratanio, "with mirth and laughter let old wrinkles come." 

Again, Shakespeare might have understood or imagined, accurately, that this is what happened with certain sad people who cannot be roused from their condition. But it has for me the feeling of lived experience. Because it is what happens. Loved ones and other kind people see you in what looks like sadness and try to help you. They want you to feel better and so they do for you the things that for them would lift them out of a bad mood or a melancholy day. But it doesn't work the way they had hoped it would.

Antonio's friends try to show understanding, try to help him make sense of what he is feeling. They suggest to the importing-exporting merchant from Venice that he is worried about his ships at sea, laden with his goods. But he tells them he is not worried. Only Salerio, inadvertently, says something that rings true. "Your mind," he tells Antonio, "is tossing on the ocean." Salerio means that Antonio is imagining his ships in peril. He's wrong, but as a description of Antonio's state of mind, probably he is not far off the mark. One way or another, Shakespeare knew.