I recently heard it claimed that objectivity in science requires direct measurement of all variables of interest, because we can only validate an indirect measure, or estimate, of some variable by comparing it to direct measurement; without such calibration, a proxy measure is empirically meaningless. I think the latter claim is false (and hence, so is the first). Suppose that in some empirical situation we have reason to believe there is a natural quantity X, the value of which we cannot measured directly, but we have some proxy measure Y which we hypothesise is proportional to X. We also find that the value of Y correlate with directly measurable variables A, B and C, which are believed, on sound theoretical grounds (i.e., parsimoniously consistent with all relevant evidence), to be determined by X. I think this would justify taking Y to be a useful estimate of X, absent evidence to the contrary. I suspect that there are many examples of this kind of reasoning in many different areas of science.
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Your specific argument in terms of X, Y, A, B, C doesn't quite work. I don't know what it means to say that "the value of Y correlates with directly measureable variables A, B, C" I think that your overall point is fine. Our measurement of many scientific quantities e.g. Avogadro's number, Planck's constant is indirect, and calculated from direct measurements. That does not mean that these measurements are infallible--both direct and indirect measurements can be erroneous. It is also worth worrying a bit about what is meant by a "direct" measurement: does it have to be something we can see unaided, or can it be something we can "see" or "detect" through an instrument? There may be no important distinction between direct and indirect measurement.
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