In the 18th century, before writing The Wealth of Nations, a fundamental text for the study of economics, Adam Smith published a remarkable book on ethics and philosophy called The Theory of Moral Sentiments. Like other Enlightenment thinkers, Smith studied human behaviour without distinguishing between different social sciences.
It was only later that various human activities became objects of study for distinct scientific disciplines. This is partly a consequence of the necessary specialisation that occurs in any field when knowledge progresses. Yet the social sciences study the same individuals and the same social groups.
Economics, for example, built its identity in the 20th century through an emphasis on statistical and quantitative methods, on one hand, and the concept of homo economicus on the other. Homo economicus is supposed rationally to defend his or her interests, given the available information.