The science of man (or the science of human nature) is a topic in David Hume's 18th century experimental philosophy A Treatise of Human Nature (1739). The science of man expanded the understanding of facets of human nature, including senses, impressions, ideas, imagination, passions, morality, justice, and society.
The science of man first established that impressions from the senses, and memories of impressions, are the foundation of all ideas. Passions are a part of human nature and they rule over our reason in determining our actions. Morality is based on necessary actions, those we make in reaction to a certain set of circumstances, and is therefore natural. However, morality is founded on self-interest, which includes the pleasure we find in sensing the pleasure in others.
Hume identifies sympathy as a passion that causes us to feel for other humans because of their similarity to us. This includes our tendency to feel, to some degree, emotions that we observe in other humans. Man has been naturally inclined to develop the rules of justice over time in order to maximize pleasure. Given the tendency of self-interest to overpower the pleasure felt through sympathy, the eventual accumulation of wealth necessitated the development of some form of government, initially somewhat monarchical, to ensure that the rules of justice were followed. Hume bases his further discussions of humans as individuals and in society in A Treatise of Human Nature on the initial premises set by his science of man.