Unobservables

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Unobservables are entities whose existence, nature, properties, qualities or relations are not directly observable by man. In philosophy of science typical examples of "unobservables" are atomic particles, the force of gravity, causation and beliefs or desires. However, some^[who?] philosophers also characterize all objects — trees, tables, other minds, microbiological things and so on to which humans ascribe as the thing causing their perception—as unobservable.

"Unobservables" is a reference similar to Immanuel Kant's distinction between noumena (things-in-themselves, i.e., the raw things before passing through the senses and the mind becoming perceived objects) and phenomena (the perceived object). According to Kant humans can never know noumena; all that humans know is the phenomena. Kant's distinction is similar to John Locke's distinction between primary and secondary qualities. Secondary qualities are what humans perceive such as redness, chirping, heat, mustiness or sweetness. Primary qualities would be the actual qualities of the things themselves which give rise to the secondary qualities which humans perceive.^{[citation needed]}

The ontological nature and epistemological issues concerning unobservables is a central topic in philosophy of science. The notion that a given unobservable exists is referred to as scientific realism, in contrast to instrumentalism, the notion that unobservables such as atoms are useful models but don't necessarily exist.

Metcalf^[1] distinguishes three kinds of unobservables. One is the logically unobservable, which involves a contradiction. An example would be a length which is both longer and shorter than a given length. The second is the practically unobservable, that which we can conceive of as observable by the known sense-faculties of man but we are prevented from observing by practical difficulties. The earth before it existed would be an example. The third kind is the physically unobservable, that which can never be observed by any existing sense-faculties of man.