 Second, although Mill never seeks in turn to provide a justification for the Principle of Transitivity of Co-existence, it is not difficult to work out what for him such justification would have to look like. The parallel case of geometric axioms is clear enough: ‘It remains to inquire, what is the ground for belief in axioms — what is the evidence on which they rest? I answer, they are experimental truths; generalisations from observation’ (1843, 231). For Mill all general propositions must ultimately be justified by experience: