20 Things You Didn't Know About Inception

Early in the film when Saito asks Cobb and Arthur if it's possible to perform inception in another's mind, Arthur tells him that it's not possible because the mind can track the source of the idea that was planted in their heads.

To demonstrate this, Arthur tells Saito not to think about elephants, which of course is what Saito does, and he knows where he got the idea to think about elephants.

There's a reason why Arthur specifically uses elephants as an example. It's a reference to the title of the book Don't Think of an Elephant by George Lakoff.

That book describes conceptual framing, the use of certain words to insert certain ideas about a subject into the listener's mind. This is exactly what Arthur tries to explain to Saito regarding inception.