Philosophy of Chemistry

What it asks

Chemistry is standardly described as applied physics. If that were straightforwardly true, chemistry's concepts would be shorthand for physical ones and its philosophy would be a footnote.

It is not straightforwardly true. Molecular structure - the shape of a molecule, which is what most chemical explanation runs on - is very hard to recover from quantum mechanics without putting it in by hand. The chemical bond, the single most useful concept in the science, has no clean quantum-mechanical definition. Orbitals are taught everywhere and are, strictly, features of an approximation.

So the branch asks what a mature science is doing when its central concepts are indispensable, accurate, and not derivable from the theory beneath it.

The recurring problems

Reduction. Chemistry is compatible with quantum mechanics and not derivable from it in practice. Whether that is a limitation of our mathematics or a fact about the relation between the two is the field's central question, and it is the sharpest real example anywhere of the reduction problem.

Molecular structure. A molecule's shape is essential to every chemical explanation. In a strict quantum treatment of an isolated molecule, the nuclei have no definite arrangement. Something has to be added to get the shape back, and what that something is remains disputed.

Models and idealisation. Chemistry runs on models known to be false and known to work - orbitals, resonance structures, ball-and-stick diagrams. That makes it the best place to study what an idealisation is doing when it succeeds.

The periodic table. Is it a discovery of a natural order or an extremely good filing system? The answer bears directly on natural kinds, and chemistry supplies the strongest candidates anyone has.

How it developed

Chemistry was almost entirely absent from twentieth-century philosophy of science. Physics was the paradigm, biology arrived in the 1970s, and chemistry was assumed to have been dealt with by the physics.

The assumption rested on the claim, made in the 1920s and repeated ever since, that quantum mechanics had reduced chemistry in principle. The phrase in principle was doing all the work, and nobody examined it.

The field formed in the 1990s, with its own journals arriving at the end of that decade, largely through chemists and philosophers noticing together that the reduction had never actually been performed.

What it found was more interesting than a negative result. The relation between chemistry and physics turns out to involve approximations that are not corrections to an exact treatment but the only route to the concepts chemists use - which has made the field a source of arguments about emergence that are grounded in a real case rather than in thought experiments.

The positions within it

Reductionism and emergentism about the chemistry-physics relation; realism and instrumentalism about orbitals and bonds; natural kind realism about the elements. All are in the Schools section.

Where it stands now

Still small, and growing, and disproportionately useful to the rest of philosophy of science because its central case is concrete.

The most transferable current work is on idealisation: chemistry uses false models successfully and systematically, and understanding how is relevant everywhere models are used, which is now everywhere.