The philosopher Charles Sanders Peirce used the term "index" to describe things like Ellie's note, things that stand as physical remnants of the causes of their existence. A footprint is the index of a foot, just as a scrape on a fender might be the index of another car or of a guardrail. Ellie's writing, a sign of her physical being, connects Carl to her through this sort of relationship.

In fact, the film characterizes Carl in part through a collection of indexical objects. His most prized remembrance of Ellie is an old photograph; many theorists and philosophers of film have claimed photography and motion pictures to be the pinnacle of indexicality, based on the relationship between the subject of a photograph and the resulting image. The scrapbook, full of photographs and other memorabilia, reads as a kind of collective index of Ellie’s life with Carl. It may also be worth noting here that Carl steers his house with a weathervane, which Peirce himself famously identifies as evidence of the blowing wind.