My Theory of Limbo
Even accepting 2SOT is not enough. There is a need to come out with a more specific definition of limbo that would replace the generic definition I mentioned earlier.
This what I propose: Limbo is a dream construct where a person connected to a dream machine would be trapped in if he or she dies in a one-layer deep dream while still very heavily sedated in reality.
If you notice, the key word here is “one-layer deep dream”. Based on examples in the film and logical reasoning, I will explain why this theory works. The mention of limbo in the film first surfaced at the “van level”. It was the scene when Eames suggested killing the wounded Saito to put him out of his misery, only to be stopped by Cobb who explained the consequences of that action by bringing in the concept of limbo.
Technically, one could kill a teammate in a deeper dream level to bring him back to a shallower level. However, in this case, killing Saito would not work because Yusuf’s sedative, which was meant to keep the team sedated for dreams that were three levels deep, was too powerful. Cobb explained that Saito would not wake up at the “airplane level”, and that his mind would go into limbo, a place where the mind would be stuck in until the sedative wore off. This could mean a perceived eternity for whoever who was trapped in limbo. By the time the person woke up, his mind would be so messed up that he would have lost sense of reality. Cobb then advised the team to proceed with the mission as planned, which was to go into the deeper dream levels.
Why is the key word “one-layer deep dream” so crucial to the understanding of limbo?
The only way to enter limbo is to die in a “one-layer deep” dream under very powerful sedation in reality. Case in point: Cobb and Saito were the only ones to enter limbo during the heist (inception) sequence. We saw that Saito died at the “snow level” but this was because he died from his wounds at the “van level (one-layer deep)” which affected his dream selves in the deeper levels. When a person dies in a dream, a logical consequence would be that he could no longer dream of a deeper level. Thus, Saito’s mind would have nowhere else to go except into limbo.
As for Cobb, he drowned at the “van level (one-layer deep)” after the van crashed into the river, bringing him into limbo. When Arthur wanted to save Cobb, Ariadne gestured to him to let Cobb drown so that he could go into limbo. Cobb expressed this rather vaguely to Ariadne at the “building level” saying that he needed to find Saito and that he was “somewhere here”.
Contrary to popular belief, Cobb did not go into limbo because Mal stabbed him at the “building level”. If Cobb died from his stab wounds, he would merely wake up at the “snow level” (provided that he died quickly enough and that the snow fortress had not exploded yet). However, based on the “slow time” theory, it would be logical to infer that the snow fortress would have exploded way before Cobb could die from his stab wounds at the deeper level. Therefore, the argument that Cobb went into limbo because he died at the “building level” is nowhere as sound as the argument that he drowned at the “van level”.
A person cannot enter limbo if he dies in a dream within a dream i.e. at least two levels deep. Case in point: At the “building level”, Ariadne and Fischer jumped to their deaths only to wake up at the “snow level”. Both of them did not enter limbo, but merely returned to a shallower dream level. Based on this reasoning, if Arthur killed himself at the “hotel level”, he would wake up at the “van level”. Similarly, if Eames killed himself at the “snow level”, he would wake up at the “hotel level”; if Eames killed himself again at the “hotel level”, he would wake up at the “van level” too. However, if both of them killed themselves at the “van level”, they would go into limbo.
Why this was so was because the powerful sedative as devised by Yusuf was used only in reality. It was not brought into any dream level to be administered accordingly. If the sedative was brought into the different dream levels, it would be pointless to use because it would not allow the team to wake up from a deeper dream level when the time came. Instead they had to wait for weeks or even months for the sedative to wear off (“slow time” theory), running the unnecessary risk of entering limbo from the deeper levels.
Instead, the sedative used in the dream levels was an ordinary (read: mild) one which would cause a person to wake up in a shallower level when a “kick” was given. This was important because Cobb’s team had planned a system of synchronized “kicks” that would allow them to ride down from the “snow level” to the “hotel level” to the “van level”, where they would wait for the powerful sedative to wear off in reality. Another possible explanation which is slightly far-fetched in my opinion is that the sedative used in the dreams were imagined. The team imagined they were sedated so that could enter a deeper level of dream.