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Natural Laws/NG/Bigelow/Pargetter: fall into two groups:
a) causal laws: they govern forces.
b) non-causal laws: they do not describe relations, but can provide indirect information about forces. Every description of the world imposes restrictions on causal relations.
For example: Kepler's laws of planetary motion: they show us which laws cannot be a correct description! They rule out earlier theories.
Preservation Laws/Bigelow/Pargetter: E.g. Law of Inertia: explains how, but not why. Indirect indications of the causes are given. For example, when acceleration is observed. However, it does not explain why a body moves at a constant speed if it is not influenced by forces.
For example, optical laws: provide even less causal information: e.g. Snell's law: (> refractive index, two materials of different density through which a beam of light moves): the height of a point above the surface is equal to the depth of the corresponding point below the surface, multiplied by a constant. It describes how light behaves, but not why it does so.
On the other hand:
For example, the law of the least time (> refractive index): between a point above and a point below the entrance area, the light takes the path for which it needs the least time. Assuming that the light moves more slowly in glass than in the air, for example, the light beam takes exactly the same path that Snell's law predicts.
Snell's law is thus subsumed under a more general law. Or it is derived from it.
Explanation: the law of the least time explains more than just refraction, it explains the path. And the law of reflection (entrance angle = exit angle). Does it also explain Snell's law?
In a sense, yes: it tells us more about the cause of refraction. But still it does not tell us why the light behaves in this way. It seems to explain it when it says that the light takes this path, "so that it" covers the distance in the shortest time. But no one understands that as an indication of a purpose._____________Explanation of symbols: Roman numerals indicate the source, arabic numerals indicate the page number. The corresponding books are indicated on the right hand side. ((s)…): Comment by the sender of the contribution.
J. Bigelow, R. Pargetter
Science and Necessity Cambridge 1990