A typical textbook explanation of a rocket’s motion when its engine is fired appeals to momentum conservation: the rocket accelerates forward because its exhaust accelerates rearward and the system’s momentum must be conserved. This paper examines how this explanation works, considering three challenges it faces. First, the explanation does not proceed by describing the forces causing the rocket’s motion. Second, the rocket’s motion has a causal-mechanical explanation involving those forces. Third, if momentum conservation and the rearward motion of the rocket’s exhaust explain why the rocket accelerates forward, then presumably momentum conservation and the rocket’s forward motion likewise explain why the rocket emits exhaust rearward. Explanatory circularity threatens to follow from this pair of explanations. This paper examines how the conservation-law explanation works and how it is compatible with the causal-mechanical explanation. The paper argues that these two explanations do not explain precisely the same fact relative to the same contrast class. The paper interprets the two conservation-law explanations as non-causal and argues that they yield no explanatory circularity.

Einstein is widely understood as regarding “principle theories” (such as the theory of relativity) as explanatorily powerless. This brief paper shows that Einstein's remarks admit of another interpretation, according to which principle theories possess explanatory power. This interpretation is motivated primarily by showing that James Jeans made remarks very similar to Einstein's at nearly the same time, but Jeans reconciled those remarks with holding principle theories to be explanatory. Einstein's remarks could well be getting at the same point as Jeans's. This view of principle and constructive theories is independently valuable. It undermines Salmon's “friendly physicist” example as an argument for the view that there are facts that can be explained by both principle and constructive theories.