Definition
A good explanation is one that is hard to vary — any alteration to its details would make it fail to explain the phenomenon it targets. Good explanations also have reach: they explain more than they were designed to explain, applying to cases beyond their origin.
Why It Matters
A ‘good explanation’ is the most powerful tool in the human arsenal because it is hard to vary; by seeking theories that are tightly constrained by reality, we gain the ‘reach’ needed to predict and control phenomena far beyond our original observations.
Core Concepts
- Hard to vary: The criterion distinguishing good from bad explanations. A myth explaining the seasons by a goddess’s mood can be varied arbitrarily without losing explanatory force — it is therefore a bad explanation.
- Reach: Newton’s gravitational law was designed to explain planetary orbits, but it also explains tides, projectile motion, and stellar collapse. This excess explanatory power is not coincidental — it signals the explanation has captured an underlying regularity.
- Science as explanation-seeking: The goal of science is not primarily prediction but the quest for good explanations. Prediction is a byproduct.
- Popper’s criterion: Falsifiability is a necessary but not sufficient condition. What matters most is whether the explanation is genuinely hard to vary.
- Heliocentric vs. geocentric: The heliocentric model (with elliptical orbits) is a good explanation — changing any detail breaks it. The geocentric model (with epicycles) could be adjusted indefinitely to fit new data, making it a bad explanation regardless of its predictive accuracy.
- Bad explanations: Easy to vary, often ad hoc. Myths, instrumentalist rules of thumb, and oracular prophecies are all bad explanations.