- For the episode, see "The Constant".
- "Variable" redirects here. For the episode, see "The Variable".
Daniel Faraday explains that when a consciousness travels back and forth through time, it needs a constant to latch on to. A constant is an object or person that exists in both periods of time, that the traveler deeply cares about and could recognize. If a constant is not found, the oscillations between different times will become more frequent and chaotic until the individual dies from what appears to be a severe brain aneurysm. Faraday's lab rat, Eloise, experienced this fate when it was unable to recognize the difference between the present and the future. Minkowski seems to have experienced the same fate and appears to have died aboard the freighter, and it is implied that the same thing happened to Brandon. ("The Constant")
- Desmond chose Penny as his constant; he was able to reach her in both times. ("The Constant")
- Faraday chose Desmond as his constant. ("The Constant")
In 1977, Daniel mentioned constants in reference to an unrelated concept involving variables. He falsely theorized that free will can alter the past with a sufficiently large action. ("The Variable")