E Direct and Indirect Measurements

A more-direct experimental test involves fewer assumptions about what the test measurements show, fewer and simpler transductions between the phys­ical properties of what is measured and the output of the measuring device, and fewer transformations of the data needed to interpret the test.

Here's an example: Nobel Prize-winning neuroscientists David Hubel and Torsten Wiesel at first studied the mammalian visual system by recording the electrical activity of individual neurons in the brain while shining a light into (anesthetized) cats' or monkeys' eyes and mapping the locations of the neurons that were activated by each eye.¹⁵ After years of tedious effort, analyzing hun­dreds of cells one at a time and collating the results, they identified patterns (the “ocular dominance columns”) in the arrangement of the responsive neurons in the visual cortex. Discovery of these columns was a breakthrough in our under­standing of how the brain processes visual information. Somewhat later, using a dye that could travel through the nerve fibers connecting the eye to the brain, they captured in a single histological image the same patterns, now formed by millions of dye-filled neurons, that they had struggled to decipher from the electrical measurements of single neurons. “Seeing is believing,” said Hubel as he displayed the picture at a large international conference, and the audience agreed: compared with the indirect information painstakingly assembled from reams of electrophysiological data, the pictorial evidence was impressively direct.

From a loftier perspective, essentially every test a scientist makes is indirect because the things a scientist is usually interested in are invisible or inaccessible. When scientists talk about doing direct tests, they are using “direct” as a relative term—it simply means more direct than other tests.

Our hypothesis regarding acid rain was actually about the hydrogen ion con­centration in rain water, and one prediction that it made depended on the output of a pH meter. The convoluted process of testing this prediction is something to keep in mind when considering that modern science commonly uses measuring devices that are enormously more complicated than a pH meter.

With these philosophical notions in mind, we can turn to the philosopher who had the greatest influence on twentieth-century science, Karl Popper, and the philosophically minded scientist, John Platt, who added practical dimensions to Popper's ideas.

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