An unobservable (also called impalpable) is an

entity An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It need not be of material existence. In particular, abstractions and legal fictions are usually r ...

whose existence, nature, properties, qualities or relations are not directly observable by humans. In

philosophy of science Philosophy of science is a branch of philosophy concerned with the foundations, methods, and implications of science. The central questions of this study concern what qualifies as science, the reliability of scientific theories, and the ulti ...

, typical examples of "unobservables" are the force of gravity, causation and

belief A belief is an attitude that something is the case, or that some proposition is true. In epistemology, philosophers use the term "belief" to refer to attitudes about the world which can be either true or false. To believe something is to take ...

s or desires. The distinction between ''observable'' and ''unobservable'' plays a central role in

Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher and one of the central Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemology, metaphysics, ethics, and aes ...

's distinction between noumena and

phenomena A phenomenon ( : phenomena) is an observable event. The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be directly observed. Kant was heavily influenced by Gottfried ...

as well as in John Locke's distinction between primary and secondary qualities. The theory that unobservables posited by scientific theories exist is referred to as scientific realism. It contrasts with instrumentalism, which asserts that we should withhold ontological commitments to unobservables even though it is useful for scientific theories to refer to them. 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. Different notions of unobservability have been formulated corresponding to different types of obstacles to their observation.

Kant on noumena

The distinction between "observable" and "unobservable" is similar to

's distinction between noumena and

. Noumena are the things-in-themselves, i.e., raw things in their necessarily unknowable state, before they pass through the formalizing apparatus of the senses and the mind in order to become perceived objects, which he refers to as "phenomena". According to Kant, humans can never know noumena; all that humans know is the phenomena.

Locke on primary and secondary qualities

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.

Philosophy of science

The ontological nature and

epistemological Epistemology (; ), or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Episte ...

issues concerning unobservables are central topics in philosophy of science. The theory that unobservables posited by scientific theories exist is referred to as scientific realism. It contrasts with instrumentalism, which asserts that we should withhold ontological commitments to unobservables even though it is useful for scientific theories to refer to them. The notion of observability plays a central role in constructive empiricism. According to van Fraassen, the goal of scientific theories is not truth about all entities but only truth about all observable entities. If a theory is true in this restricted sense, it is called an ''empirically adequate'' theory. Van Fraassen characterizes observability counterfactually: "X is observable if there are circumstances which are such that, if X is present to us under those circumstances, then we observe it". A problem with this and similar characterizations is to determine the exact extension of what is unobservable. There is little controversy that regular everyday-objects that we can perceive without any aids are observable. Such objects include e.g. trees, chairs or dogs. But controversy starts with cases where unaided perception fails. This includes cases like using telescopes to study distant galaxies, using microscopes to study bacteria or using cloud chambers to study positrons. Some philosophers have been motivated by these and similar examples to question the value of the distinction between observable and unobservable in general.

Kinds of unobservables

W. V. Metcalf 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 third kind is the physically unobservable, that which can never be observed by any existing sense-faculties of man.

References

{{metaphysics Concepts in epistemology Concepts in metaphysics

Kant on noumena

Locke on primary and secondary qualities

Philosophy of science

Kinds of unobservables

See also

References