Warning. Contains some spoilers for season 3 of Breaking Bad.
Contamination is the adulteration of something perfect. Purity is perceived to have been compromised. Walt gets an obsession with a fly that he sees as ruining the chemical sanctity of his otherwise pristine laboratory, and he spends all night trying to track it down and kill it. His knowledge of science should remind him that human reflexes are rarely sufficient in this situation, but the obsession wins out and in the end Jesse has to dose him with sleeping pills without his knowledge to get him to rein in his thoughts.
The possum under the floor occurs to Jesse because that’s what his Aunt became obsessed with when the cancer had spread to her brain. She even gave it a name. Walt is in remission though, the fly is there and he’s chosen to tip his existence into something not sustainable. Not only is the bar set too high: it’s a bar that’s too tiny and insignificant for most people to see. And that’s what sustainable is. It’s about not letting a fly gain dominion over your entire existence. Because while you’re obsessing about a fly the chances are that a brontosaurus will lumber out of nowhere and stamp on your head.
The possum under the floor undermines you, in the same way as the weasel under the cocktail cabinet was said to undermine characters in Pinter’s early plays. Possums and weasels build empires when you’re not looking, however. By the time you’ve had your eyes focused in the wrong direction for a few years they can make themselves into formidable adversaries. When Walt speaks of the “perfect moment” he’s imagining a time when he can die from his lung cancer and he will have left exactly the legacy he is looking for. Meanwhile, in the real world, there is no perfect moment. Anthony Hopkins once explained his decision to give up drinking in terms of waiting for the “big event”. He realised that this, here, now is the big event.
Make sure that you build carefully. Listen for the scratching of the possum beneath your new floorboards, and leave a hatch with a well oiled hinge so that you can negotiate its peaceful exit. Don’t buy a cocktail cabinet with a hidden compartment that allows the weasel to consume your entire stock. You never know when you might need one.