PT158 · S2 · Q19 The differences in distance between

This answer claims it is "not possible" to determine brightness differences between stars in different galaxies. The argument does not need this extreme claim. The argument only needs studying same-galaxy stars to be a viable method — not that it is the only possible method. Even if cross-galaxy brightness comparisons were perfectly feasible, the argument's conclusion (that same-galaxy study can reveal correlations) would still hold. The argument advocates for one research approach; it does not need to rule out all others. Moreover, the argument's core vulnerability is about non-brightness variation, not about the exclusivity of its proposed methodology.

Whether differences in the elements each star burns account for differences in brightness is a specific causal claim about what produces brightness variation. The argument does not need to assume any particular cause of brightness differences — it only needs brightness differences to exist and to be measurable (which it established through the distance-control reasoning). The conclusion is about correlating brightness with other characteristics, not about explaining why stars have different brightnesses. Furthermore, the "other characteristics" in the conclusion are characteristics that could be correlated with brightness, not necessarily the causes of brightness. This answer addresses a causal question the argument never raises.

Whether the stars in our own galaxy are at approximately the same distance from Earth is irrelevant to the argument's reasoning. The argument specifically discusses distant galaxies, where the enormous distance from Earth makes intra-galaxy distance differences negligible. Our own galaxy — the Milky Way — is a different case entirely because we live inside it, so distance differences between stars are very significant. The argument never claims or needs to assume anything about the distance distribution of stars in our galaxy. Its methodological proposal is specifically about distant galaxies, and conditions within our own galaxy are outside the scope of the argument.

This answer describes a relationship that is actually consistent with the argument's reasoning rather than being an assumption the argument requires. The argument already establishes that distance differences affect apparent brightness — that is the very reason it proposes studying same-galaxy stars (where distance is controlled). This answer essentially restates a consequence of the argument's own premises rather than identifying an unstated assumption needed to reach the conclusion. The argument needs something about non-brightness variation, not a restatement of the distance-brightness relationship it already acknowledged. Moreover, this answer does not pass the Negation Test in a way that undermines the conclusion: negating it would mean distance differences do NOT cause apparent brightness differences, which would actually undermine the premise (not the unstated assumption) of the argument.