These are some theories that make Inception work; some are described in detail in the film by Nolan, a few others are inferred from the way the film works, and the rest are suggested based on my observations.
ð “Pain” theory – it is suggested by Nolan that feeling pain in the dream is as painful as that of reality. But when a person wakes up from that dream into a shallower dream level, or reality, the pain disappears. In addition, pain becomes less intense when a person goes into a deeper dream level.
ð “Dream machine” theory – the team is plugged into the dream machine intravenously. It filters everyone’s subconscious into that of a single chosen individual i.e. the host. It could be programmed with different dream levels, and a time period is pre-determined for the sedative to function. The sedative will continue to be administered to each person until the pre-determined time period ends, waking all of them up.
ð “Safe” theory – it is suggested by Nolan that a safe or vault could be created in a dream to store and protect one’s own secrets.
ð “Waking up” theory – there are THREE ways for a person plugged into a deeper dream level via the dream machine to wake up in a shallower level: One, he is “kicked” at the shallower level by the host; Two, he dies at the deeper level; Three, the timer on the dream machine runs out i.e. the sedative is not administered anymore.
ð “Totem” theory – a totem is a personal object with unique properties that would indicate to its owner whether he is in a dream or reality. E.g. Cobb’s totem is a top. It spins forever in a dream, but it will wobble and topple in reality.
ð “Slow time” theory – when a person enters a deeper level of dream, time seems to move slower.