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The concepts of Horizontal Equity and Vertical Equity were coined by the economist Richard A. Musgrave in the 1950s. Given a tax base, such as income or consumption, Horizontal Equity (HE) requires that people in equal positions should be treated equally, whereas Vertical Equity (VE) considers how differently should people in different positions be treated. This paper reconstructs the genealogy of Musgrave’s dual concepts of fiscal equity. It shows how a concern for tax equity has been central to philosophers and economists from Hobbes to Smith, to Musgrave. I show that HE and VE are fiscal applications of the Aristotelian formal concept of distributive justice. This suggests a role for the principle of HE in the context of non-ideal political debates about tax reform. HE can be used to flag morally unacceptable forms of discrimination.JEL Codes: H20, B11, B12, B20, B40, D63.