Many of the most prominent researchers in the field of psychology (including William James, Sigmund Freud, Carl Jung, Jean Piaget and Lev Vygotsky) have viewed play as endemic to the human species. Play may be frivolous; but it may be serious. A player can be intently focused on his or her objective ("in the zone"), particularly when play is structured and goal-oriented, as in a game. Accordingly, play can range from relaxed, free-spirited and spontaneous through frivolous to planned or even compulsive.