Death of a Salesman/WMG

The cheese mentioned at the beginning of the play represents the American Dream.

 * Linda tells Willy that she bought a new kind of American cheese, and mentions that it is whipped.
 * The fact that it is American cheese makes it obvious that it represents the American Dream, and it is whipped because the American Dream has been "whipped" to death; nobody takes it seriously anymore.
 * It is a new kind of cheese, much like a new era has begun in which the American Dream is a thing of the past.
 * Willy mentions that he prefers Swiss cheese. Though this could be taken to mean that he has a subconscious desire to move to Switzerland, but wait just a minute. Swiss cheese, as we all know, is full of holes. Willy's ideals and goals have been riddled with holes, but he still clings to them.
 * He also wonders aloud why anyone would want to whip cheese, as he cannot understand why anyone would give up on the American Dream.
 * Later, Linda asks her husband if he ever ate any of the cheese, a question he does not answer. He is afraid to admit that he never achieved the American Dream.
 * And he never will, because the Loman family's refrigerator broke, and eventually the cheese will spoil and become unsafe to consume.

Willy Loman was Dead to Begin With.
Willy got into a car accident on the way home that night, and the person we see is actually his ghost come to say his last goodbyes.
 * Note that his first line in the play is "It's all right. I came back." Back home, or Back from the Dead?

Willy Loman has Bipolar disorder
If you look at the list of symptoms, you can check them all off.
 * That's not a WMG...that's the very point of the story.
 * I was under the impression that Willy was suffering from early-onset Alzheimers. Confusion with time and place, poor judgement, Difficulty staying in the moment, forgetting what's just been said to him, confusion of what's going on around him, it all adds up.

Willy Loman is a time traveler...
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 * ...And he just doesn't realize it. Throughout the course of the play, he's not just having flashbacks; he is quantum leaping through the events of his life. The emergence of these powers coincides with his worsening mental state, however, which is why he only flashes back to the periods in his life (where everything went wrong with Biff, when he turned down Ben's offer) where everything went wrong. The strain proves to be too much in the end, and helps drive to his suicide (...or did he simply take the final leap?).
 * Also, when he interacts with the dead Ben in the "present", it's still time travel. This time, however, Willy has gotten stuck between the two time periods.