Play is a range of intrinsically motivated activities done for recreational pleasure and enjoyment. Play is commonly associated with children and juvenile-level activities, but may be engaged in at any life stage, and among other higher-functioning animals as well, most notably mammals and birds. Many prominent researchers in the field of psychology, including Melanie Klein, Jean Piaget, William James, Sigmund Freud, Carl Jung and Lev Vygotsky have erroneously viewed play as confined to the human species, believing play was important for human development and using different research methods to prove their theories. Play is often interpreted as frivolous; yet the player can be intently focused on their objective, 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. Play is not just a pastime activity; it has the potential to serve as an important tool in numerous aspects of daily life for adolescents, adults, and cognitively advanced non-human species (such as primates). Not only does play promote and aid in physical development (such as hand-eye coordination), but it also aids in cognitive development and social skills, and can even act as a stepping stone into the world of integration, which can be a very stressful process. Play is something that most children partake in, but the way play is executed is different between cultures and the way that children engage with play varies universally. Definitions The seminal text in the field of play studies is the book Homo Ludens first published in 1944 with several subsequent editions, in which Johan Huizinga defines play as follows: This definition of play as constituting a separate and independent sphere of human activity is sometimes referred to as the "magic circle" notion of play, a phrase also attributed to Huizinga. Many other definitions exist. Jean Piaget stated, "the many theories of play expounded in the past are clear proof that the phenomenon is difficult to understand." There are multiple aspects of play people home in on when defining it. One definition from Susanna Millar's The Psychology of Play defines play as: "any purposeful mental or physical activity performed either individually or group-wise in leisure time or at work for enjoyment, relaxation, and satisfaction of real-time or long term needs." This definition particularly emphasizes the conditions and benefits to be gained under certain actions or activities related to