What is established a posteriori is merely that H2O is the substance that underlies and explains waterish properties, not that the sufficiency of the base requires a posteriori reasoning
Reasoning where the transition from premises to conclusion makes no appeal to what we learn from observations
substance(Spinoza's metaphysics; criteria include (i) necessity and (ii) self-subsistence)
The fundamental existent that is wholly necessary and self-subsistent, not depending on anything else for its existence
sufficiency of the base(whether the basic materials alone explain water's properties or if we need other kinds of knowledge)
Whether the fundamental building blocks (like atoms) are enough by themselves to explain everything without needing additional reasoning.
waterish properties(Used to illustrate the relationship between micro-structural facts and manifest phenomena)
The manifest, macro-level observable properties associated with water (e.g., transparency, liquidity, quenching thirst), as distinguished from the micro-level chemical properties of H2O
The only general argument that seem to be available for this would be the principle that, for any two levels of discourse, A and B, they are more-than-causally connected only if one entails the other a priori. And the argument for accepting this principle would be that the relatively uncontroversial cases of a posteriori necessary connections are in fact cases in which one can argue a priori from facts about the microstructure to the manifest facts. In the case of water, for example, it would be