This Wired article discusses films that influenced George Lucas. According to the article:

"The film that made the most profound impression on Lucas, however, was a short called 21-87 by a director named Arthur Lipsett, who made visual poetry out of film that others threw away. Working as an editor at the National Film Board, he scavenged scraps of other people's documentaries from trash bins, intercutting shots of trapeze artists and runway models with his own footage of careworn faces passing on the streets of New York and Montreal."

The article explains that:

"One of the audio sources Lipsett sampled for 21-87 was a conversation between artificial intelligence pioneer Warren S. McCulloch and Roman Kroitor, a cinematographer who went on to develop Imax. In the face of McCulloch's arguments that living beings are nothing but highly complex machines, Kroitor insists that there is something more: "Many people feel that in the contemplation of nature and in communication with other living things, they become aware of some kind of force, or something, behind this apparent mask which we see in front of us, and they call it God."

When asked if this was the source of "the Force," Lucas confirms that his use of the term in Star Wars was "an echo of that phrase in 21-87."