What are the evidences that show the whole film (Inception) is a dream?
1. Repeated lines of dialog shared amongst the characters (Mal and Saito both tell Cobb to take a "leap of faith", Cobb predicts what Saito will say in limbo).

2. Numerous improbable or coincidental events that happened during parts of "reality". Cobb is saved by Saito coincidentally during the chase between Cobol Engineering agents and him, Cobb squeezing through the walls during the chase, to what certain extent the power that Cobol Engineering and Saito possess, etc.

3. The line from the old man in the underground lab of Yusuf: "Their dream has become their reality. Who are you to say otherwise?" or even Miles line: "Come back to reality, Cobb", or even James (Cobb's son) when Cobb asked: "What have you been painting? and he said at the end of the film: "We're building a house on a cliff".

4. We never get to see Ariadne or Arthur use their totem. Why so?

5. The clumsiness of the homicide police investigation regarding Mal's case and put the blame on Cobb entirely.

6. The spinning top at the end of the film. The top is a totem used to help verify whether Cobb's in reality or in someone else's dream, but it offers no verification against being in his own dream. He knows the exact weight, composition and how it should spin. So whether the top stops or continue spinning, it's not important as even when it stops, it could be that Cobb believes that he finally reunite with his family, hence his dream fully becomes his reality.

7. In Mombasa, in the bathroom after he tests Yusuf's sedative, you see the figure of Mal behind the curtains, if he was in "reality" then his subconscious could not be projecting her.

8. Cobb's totem was not Mal's top, rather one could suggest that Mal was his reality check, his 'real' totem. Yet throughout the movie he was directly or indirectly responsible for either killing her or imprisoning her, in essence losing his sense of reality and refusing to face up to the facts - which may be that he was indeed dreaming the whole time.

9. The musical score that is heard, is the slowed down playhead of "Non, Je Ne Regrette Rien", which is the Edith Piaf song used for the musical countdown. The significance here is that the deeper you go into the dream, time slows, thus the music will slow too, mimicking the score. This musical design perhaps indicates that Cobb's reality was still very much a dream state too.

Alternatively,' the whole film is a dream' in the sense that Nolan is suggesting that what we think of as a waking state, or 'reality', is in fact just more dreaming.

Hinduism and Buddhism both share the idea that the world is 'maya', or an illusion and that we are on a journey of 'awakening from the dream'. This has been taught by such Indian saints as Sri Ramana Maharishi and Maharishi Mahesh Yogi, who call it 'Advaida', which means non-duality; the idea that all the universe is just one consciousness, connecting everything in it. We act out our lives like a play shone onto a movie screen. But only those who achieve Nirvana (a.k.a enlightenment, or becoming one with Nature) can perceive the illusion for what it is (a dream) and become aware of the 'screen' on which the great drama is unfolding.

According to the Advaida philosophy, it is only when we are in a state of deep sleep that we come close to the timeless, 'true' state of being that is the universal consciousness, which all Nature is part of. When we wake up, in effect we are waking into a world that is illusory.

A separate movement believes in the specific idea that humanity is collectively dreaming a dream of guilt and separation, from which we need to wake up by practising forgiveness. This is a metaphysic ideology given in A Course in Miracles and The Way of Mastery, which are spiritual texts purportedly channelled from Jesus in the 1970s and 1990s respectively. ACIM in particular has influenced most spiritual 'new age' authors and commentators today.