This chapter explores how Marx combines philosophical and economic research, developing the concept of alienation into the theory of alienated labor, which he uses to critique bourgeois political economy. During his time in Paris, Marx conducts an in-depth study of economics, analyzing phenomena such as private property, money, and credit, thereby revealing the alienated nature of labor in capitalist society and its distortion of human and social relations. Marx argues that alienated labor is the root of private property. In capitalist society, labor is reduced to a mere means of subsistence, human relations are replaced by relations between things, and money becomes an alienating force that dominates everything. Additionally, Marx distinguishes between “alienated society” and “true society.” The former is based on private property, where social connections manifest as the externalization of exchange value; the latter aligns with the human essence, where production serves to meet human needs. Finally, Marx analyzes the alienating effects of money and credit. This chapter demonstrates that the theory of alienated labor represents a crucial turning point in Marx’s early thought, serving both as a critique of bourgeois economics and as a scientific attempt to explore the laws of history. Although its methodology still has limitations, it lays the theoretical foundation for revealing the contradictions of capitalism and the path to human emancipation.