Social justice is a controversial notion that emerged in political debates in the nineteenth century. It subsequently began to interest and be used by economists, but its meaning crucially depends on the theory of law that supports it. This concept has been particularly developed by social Catholicism based on Aquinas’ natural law. After a rapid account of the diversities in the meaning of justice in relation to the specific philosophy of law, we present Taparelli’s development of the notion of social justice. We then highlight some of the differences in the use of this concept in early social Catholicism to point out the difficulties it implies in a context dominated by a positive idea of law.