Maslow's hierarchy of needs is a theory in psychology proposed by Abraham Maslow in his 1943 paper "A Theory of Human Motivation" in Psychological Review. Maslow subsequently extended the idea to include his observations of humans' innate curiosity. His theories parallel many other theories of human developmental psychology, some of which focus on describing the stages of growth in humans. He then created a classification system which reflected the universal needs of society as its base and then proceeding to more acquired emotions. Maslow's hierarchy of needs is used to study how humans intrinsically partake in behavioural motivation. Maslow used the terms "physiological", "safety", "belonging and love", "social needs" or "esteem", and "self-actualisation" to describe the pattern through which human motivations generally move. This means that in order for motivation to arise at the next stage, each stage must be satisfied within the individual themselves. Additionally, this theory is a main base in knowing how effort and motivation are correlated when discussing human behaviour. Each of these individual levels contains a certain amount of internal sensation that must be met in order for an individual to complete their hierarchy. The goal in Maslow's theory is to attain the fifth level or stage: self-actualisation."