Despite the intense interest in the phenomena of presence, there have been limited attempts to explain the fundamental reason why human beings can feel presence when they use media and/or simulation technologies. This is mainly because previous studies on presence have focused on “what” questions—what are the causes and effects of presence?—rather than the “why” question. The current paper tries to solve this problem by providing an elaborated—and probably controversial—account of the fundamental presence-enabling mechanism. More specifically, it explains the modularity of human minds, and proposes that human beings can feel presence due to the automatic application of two types of causal reasoning modules—folk-physics modules for knowing about physical causation, and folk-psychology modules for knowing about social causation—when they respond to mediated and/or simulated objects. Finally, it explains the media-equation phenomena (in which media or computer users feel physical or social presence) according to the modularity argument.