The Wikipedia page on unobservables states that:

There is considerable disagreement about which objects should be classified as unobservable, for example, whether bacteria studied using microscopes or positrons studied using cloud chambers count as unobservable.

Could somebody please summarise this debate for me? The page doesn't have a reference. What are the arguments in support of each side of this disagreement?

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    Yes, according to constructive empiricists like van Fraassen, see SEP:"as a constructive empiricist would use the terminology, one only observes something when the observation is unaided. One does not see cells through a microscope; instead one sees an image, an image which the scientific gnostic understands one way but the scientific agnostic understands a different way." Husserl can be read to hold a similar view, but it was before current discussions and he used different phrasing ("immediately given", etc.)
    – Conifold
    Commented Jun 14 at 22:05
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    It seems to me there comes a point where common sense should prevail and obsessing over exact definitions serves no useful purpose. All perceptions are subject to uncertainties, so from an epistemological sense there is no real distinction between "observable by eye" and "observable with a microscope", just some additional sources of uncertainty/assumptions - but it isn't as if "observable by eye" is without uncertainty or assumptions. For me pretty much anything more than a couple of feet away is likely to be unobservable for me without my glasses - where do you draw the line? Commented Jun 16 at 14:00

2 Answers 2


See definition 24.1 on page 2 of this 12 page pdf for the definition of an unobservable state in control systems theory:


a state q of a system is unobservable if its initial condition is indistinguishable from the zero initial condition.

In economics there is a concept called the natural rate of interest and it is held to be unobservable (which is why some experts argue it is irrelevant due to non-existence):


In the long run, economists assume that nominal interest rates will tend toward some equilibrium, or “natural,” real rate of interest plus an adjustment for expected long-run inflation.

Unfortunately, the “natural” real rate of interest is not observable, so it must be estimated.

The Natural Rate of Unemployment (NAIRU) is another controversial variable which is thought to be greater than zero, but otherwise cannot be measured directly, and which could be considered unobservable:


In the context of macroeconomics, the two political problems are keeping the currency stable against episodes of inflation or deflation and fostering maximum employment using fiscal and/or monetary policy. NAIRU is rejected by some Post-Keynesian economists who argue that fiscal policy should be used to employ everyone who wants a job that fits their skills. The point I am making is that arguments over observable and unobservable variables infect politics and economics.

These concerns about unobservable states relate to our conceptual math models for physical or economic systems and not to the philosophy of classifying human perceptions using the naked eye versus instruments that extend our perceptions.

The bacteria seen under a microscope are thought to be images which are magnified by the properties of light and lenses. When there is sufficient light, and proper instrument design, we assume the images are accurate scaled representations of what is seen with the instrument due to our coherent theories of light and lenses. We make semiconductors by sending light through photomasks and lenses to reduce the size of physical features in the semiconductor structure, and we do so very effectively, incorporating the principles of chemistry, optics, and materials science. But if we speculate about what we see or otherwise detect using a microscope, telescope, or scientific instrument it raises the question of what is observable versus unobservable.


When you look at something with the unaided eye and it is too small to make out clearly, you then get a hand lens and hold it in front of your eye to make the image larger. The more powerful the lens, the smaller the object your eye can then see.

Now note that your eye already has a lens in it so that when you are looking at something with the unaided eye, you are actually observing it through the use of a lens. Now if you define an observable as something you can see without a lens, then shouldn't the no-lens observability condition apply to the eye itself?

Carrying this line of argument a little further, note that at age 72 I can no longer read or draw fine print like a 10 year old can. What is "observable" for a 10-year-old is not "observable" for me.

This would seem to make the philosophical concept of "observability" uselessly problematic...

  • ...but what a "lens" is and how it functions requires already a theory which explains it, and then you'd need to believe at least to some extend that what you describe is "real". Commented Jun 16 at 6:46
  • @JasonFunderberker which is why I am a physics guy and not a philosopher! Commented Jun 16 at 17:40
  • Right but this has the issue that the lens in our eyes is part of the perception which gives us the foundation of our knowledge. You could say that a microscope gives an extra level of indirectness which would make the bacteria an unobservable
    – edelex
    Commented Jun 17 at 16:59
  • As a philosopher, you are richly entitled to say anything you want ;-) Commented Jun 17 at 17:13
  • @edelex And if you have an artificial lens in your eye, e.g. due to cataract, has the whole world become unobservable to you, although you can actually see it better than with your natural lens?
    – Igor F.
    Commented Jun 17 at 17:51

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