The Tommy Westphall Hypothesis, or Tommy Westphall Universe, is an idea of shared universe among hundreds of television shows that are in the imaginings of young Tommy Westphall. The origin of this hypothesis is "Six Degrees of St. Elsewhere" by Dwayne McDuffe. Keith Gow and Ash Crowe took it further creating the Tommy Westphall Universe. It has grown to the rank of internet meme. The central source for adherents is The Multiverse.
Given the fame of the hypothesis, it has quite naturally gathered differing opinions. The most negative of these have created counterarguments, the most notable of which is "Six Objections to the Tommy Westphall Hypothesis".
Other opinions exist without denying the concept. This creates widely varying interpretations. Some reject cartoon crossovers, while others accept them under somewhat stricter guidelines. Most insist that the phenomenon be restricted to television, but a select few allow crossovers to branch through nearly all types of fiction.