Lyons, Kepler, and the commitments of deployment realism

Timothy Lyons argues that Psillos’ “deployment realism” should be committed to the truth of all the components actually employed in reaching successful novel predictions. He then explains that Kepler made novel predictions (e.g., that the Sun spins, that a planet’s speed is highest at its perihelion and lowest at its aphelion, etc.) reasoning from the false assumptions that (1) the planets tend to rest, but the Sun rotates, and transmits this rotation to them through rays whose force decreases with the distance. This, concludes Lyons, refutes deployment realism (T. Lyons, “Scientific Realism and the Stratagema de Divide et Impera”, BJPS 2006). I reply that in abduction we should postulate only the weakest cause sufficient to explain the effects. Equally, in explaining a novel prediction, we should assume the truth of only the essential components, i.e. the weakest ones sufficient to reach the prediction. Kepler abductively inferred (1) from facts he knew: (2) the planets move around the Sun on the same plane and in the same wise, and their velocities are in the inverse order as their distances from the Sun. This could suggest that the solar system rotates as a coherent (but viscous) disk, whose periphery is slower than the centre. Of course (1) was unnecessarily strong as an explanation of (2), but it had a weaker core, better supported by (2), and true: (3) the solar system moves around (a point close to) the centre of the Sun, due to something which is inversely related to the distance from it. Since Kepler’s new predictions could already be derived from (3), (2) was not essential to them, while (3) was essential but true, and this confirms deployment realism.