What is truth? That question is as old as thought itself but shows no sign of ageing. This paper develops the approach toward that question by Gottfried Wilhelm Leibniz (1646-1716). Leibniz accepts the truthfulness of claims of the mathematical or natural sciences, but rejects the sufficiency of that approach, saying evidently there is more to life than 'quantity'. By definition, what is 'more' than quantity will be 'as nothing' to the sciences of quantity, and it is in that sense this thesis aims to be a proof of nothing. It concerns a truth which cannot be proved in science, but only believed or not based on reasoning. For Leibniz, the final truth is the "reason why we ourselves exist rather than not". Leibniz makes his argument through his "so-called middle science [which] is nothing but the knowledge of contingent possibles". The middle science stands to bridge between science and philosophy, mathematical quantity and metaphysical being, necessity and freedom. It does so firstly as Immanuel Kant does, by justifying belief in 'how' scientific knowledge could be possible. But Leibniz rejects the sufficiency of that merely mechanical sort of account, saying there must also be a reason 'why'. In pursuing his inquiry, Leibniz revisits ancient metaphysical concepts like 'substance' and Aristotle's 'third thing', thereby producing a truly universal, "organic" account, which stands to justify belief in 'intelligence' and not just 'sense'. Leibniz's approach is postmodern because it validates the actual individual whose truth is unique and therefore as nothing to modern science. But his middle science is also better than postmodernism, and I say can be called 'post-modern', because it makes a transparent connection between science and individual, yet without sacrificing priority to the individual. It is worth considering because it is accepting of science, comprehensive of our situation, not logically contradictory and seems able to clarify concepts problematic in philosophy today, such as 'possible world semantics', 'dispositions' and 'counterfactuals'.