On the Groundwork of Empirically Informed Measures of Needs-Based Justice. Two Desiderata Between Normative Theory, Formal Modelling and Empirical Social Research. Philosophical theories of distributive justice are usually formulated in a purely verbal way. Thus it is not always clear how they are actually to be applied to different specific distribution situations. It is often not possible to say what influence, for example, small variations of distributions should have on their assessment. In order to solve this problem of vagueness, an attempt has recently been made to formalise needs-based justice by means of indices in order to obtain precise mathematical tools for assessing the justice of distributions. The chapter introduces two classes of axioms - “monotonicity” and “sensitivity” - that represent fundamental assumptions for the construction of such indices. In addition, empirical surveys with lay people are conducted on the normative considerations that guide those classes of axioms. It turns out that this provides a wider range and diversity of intuitions that can be reflected upon. It is shown how this can be fruitful for the present case of reflections on axioms of needs-based justice.