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The unificationist model proposes that explanation is not a property of individual derivations but of unified systems of derivations. A good explanation unifies diverse phenomena under few principles. Science progresses by achieving deeper unification: mechanics unifies terrestrial and celestial motion, thermodynamics unifies heat and mechanical energy, evolution unifies biological diversity. Unification captures why Maxwell's equations and Einstein's relativity count as great explanatory achievements even when they don't fit the D-N form.
You already know the deductive-nomological (D-N) model: an explanation is a valid argument from laws and initial conditions to the explanandum. If you want to explain why this match lit, you cite the laws of combustion plus the specific conditions (oxygen present, friction applied, etc.). The D-N model captured something real — explanation involves laws — but it faced persistent problems: statistical laws didn't fit the deductive structure, some valid D-N arguments felt unilluminating (why does explaining a flagpole's length from its shadow count as explaining?), and the model didn't say why some laws explain better than others.
The unificationist model, developed by Michael Friedman and refined by Philip Kitcher, shifts the unit of analysis from individual explanatory arguments to whole *systems* of explanation. The core claim is that science explains by reducing the number of independent phenomena we must accept as brute facts. A theory that derives 100 different phenomena from 3 principles provides more understanding than a theory that explains each phenomenon separately — not because any individual derivation is different, but because the unified system reveals deep connections in nature. Newton's mechanics didn't just explain falling apples; it explained tides, planetary orbits, and pendulums under the *same* framework, collapsing what had seemed like independent regularities into a single structure.
The key concept is argument patterns: Kitcher analyzed unification in terms of the number of distinct argument schemas a theory requires versus the number of phenomena it covers. A maximally unified science would cover everything with a single abstract pattern instantiated in all domains. This is why Maxwell's unification of electricity, magnetism, and light was so impressive — three apparently separate phenomena turned out to be manifestations of one set of equations. And it explains why Einstein's general relativity is considered explanatorily profound: it subsumes gravity under spacetime geometry, eliminating gravity as a separate force and reducing what counts as unexplained.
What the unificationist model adds beyond the D-N framework is an account of *why some laws explain better than others*. A law that connects to a deep structural feature of the world appears in many argument patterns; a superficial generalization participates in few. This also vindicates the intuition that theoretical reduction — showing that higher-level phenomena are instances of lower-level principles — is a genuine explanatory achievement, not merely a restatement. The model has limitations (it is difficult to formalize "argument patterns" precisely, and it may miss cases where causal information explains even without unification), but it captures something the D-N model missed: the explanatory power of broad theoretical scope.
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