Indirect but Theory-Free Observation

Theoretical hypotheses are a creation of human mind, introduced as the best explanation of certain phenomena, and tested by their observable consequences. Antirealists object that (a) any explanation is underdetermined and (b) confirmation by consequences is the invalid scheme of adfirmatio consequentis.

However, some theoretical entities can be observed by instruments which do not presuppose theories (Alai 2010: § 4). Van Leeuwenhoek, a draper, invented the optical microscope by observing that the lenses he used to ascertain the quality of his fabrics magnified small but observable objects by 200 or 300 times. Thus, when through his lenses he saw things which escaped the naked eye (like bacteria, spermatozoa, and muscular fibers), he could safely assume that those were real entities, and approximately measure their actual size. In so doing he relied merely on his eyes, the uniformity of nature (e.g., the assumption that the ratio of the lens images to their objects did not vary at different scales), and elementary mathematics.

Perrin was able to measure Avogadro's number, hence the volume and size of molecules, by procedures which also presupposed nothing but the uniformity of nature and easy computations. For instance, he dropped a droplet of oil on a water surface covered by talc powder: the drop expanded, pushing the talc aside. Even­tually it reduced to the thickness of one molecule and became invisible, but its surface coincided with the talc-free area, and could be measured. Thus, by dividing the volume of the droplet times the surface, he found the value of about 1 pp for the diameter of molecules (Perrin 1913: § 32).

Another example is the chemical composition of the stars: it cannot be observed directly, but it can be recognized by observing the same spectra which are emitted by samples of different elements in the laboratory. Probably cases like these are more common than it might seem, and in front of them stubborn antirealists must pay a high price: doubt the uniformity of nature, and give up induction in science and everyday life (Alai 2010: § 4).

Perrin's measurement of Avogadro's number also exemplifies the confirming power of Whewell's “consilience of inductions”, because he reached the same results by different methods (Kosso 1992: ch. 9). Those procedures presupposed various theories, but, since they were mutually independent, the agreement of their results would be a miraculous coincidence unless they had some real grasp of the world. So, this reasoning is just another variant of the NMA.

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