On evidence

by Neil Rickert

In a recent blog post about the scientist’s operating manual, John Wilkins asked about evidence gathering by scientists.  I commented:

What is missing in your account, is that often evidence is not readily available. Scientists often need to be creative in coming up with ways of getting evidence. And, at times, this requires being creative in finding new concepts with which the new evidence can be expressed.

So here I am, expanding on that comment.

I will start with a couple of examples, to illustrate the point.


My first example, from physics, is the Michelson-Morley experiment.  Michelson and Morley had the idea of measuring the aether drift by comparing the speed of light in two different directions.  Now that sounds simple.  We could measure the speed of light in each of those directions, subtract the two measurements, yielding the difference.  The problem, though, is that it is difficult to measure the speed of light with anything close to the precision that would have been required for such a simple approach.  So, instead, they came up with a creative plan to build an interferometer, whereby they could split a beam of light, have part travel in one direction and part in another, then recombine them in a way that wave interference could be observed.  That inventive experimental design allowed observation of a difference in speeds at a far higher precision than was available for the direct measurement of the speed of light.  The experiment showed no detectable aether drift, and this was taken as evidence against the aether theory of light.

Defining a new species

For my second example, I will use a hypothetical case from biology.  Let us suppose that there is a species, call it spX, of insects being studied.  The biologists are studying the habitat and the ethology of insects of this species.  As they proceed with their studies, they begin to realize that this should be considered to be two species rather than one.  So they name a new species, call it spY, and they provide criteria to distinguish between organisms of spX and organisms of spY.  As a result, there are now many more expressible facts.  For example, facts which state differences between the behavior of organisms of spX and those of spY are now expressible, whereas they were not expressible before the new species was defined.  As in the Michelson-Morley example, this allows the expression of information which is more precise than would have been previously possible.


The Michelson-Morley experiment is well known, and it is reasonably common for biologists to define new species.  It seems to me, however, that the epistemic significance has been largely overlooked.  Epistemology is often presented as if there exists a fixed set of true propositions, and our job is to find out what those propositions are.  What I observe, however, is that the set of expressible propositions is not fixed.  New methods of getting evidence, or new concepts (such as that of the species spY), expand the set of potentially expressible true propositions.  So there is more to acquiring knowledge than simply picking up the facts.

The example of defining as new species fits rather well with what perceptual psychologist Eleanor Gibson described in her 1969 book “Principles of Perceptual Learning and Development”.  For it is an example of a newly acquired ability to discriminate between organisms of spX and those of spY.  Gibson used the term “perceptual learning” for acquisition of such abilities.  And since the Michelson-Morley experiment also allowed for more precise observations, that could also be considered an example of perceptual learning.  Our scientific knowledge is growing in ways that are not adequately described by epistemology.

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