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 ...

whose existence, nature, properties, qualities or relations are not directly

observable In physics, an observable is a physical quantity that can be measured. Examples include position and momentum. In systems governed by classical mechanics, it is a real-valued "function" on the set of all possible system states. In quantum phy ...

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 ultim ...

, typical examples of "unobservables" are the

force of gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...

, 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 tak ...

s or

desires Desires are states of mind that are expressed by terms like "wanting", " wishing", "longing" or "craving". A great variety of features is commonly associated with desires. They are seen as propositional attitudes towards conceivable states of aff ...

. 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 ...

's distinction between

noumena In philosophy, a noumenon (, ; ; noumena) is a posited object or an event that exists independently of human sense and/or perception. The term ''noumenon'' is generally used in contrast with, or in relation to, the term '' phenomenon'', whi ...

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 John Locke (; 29 August 1632 – 28 October 1704) was an English philosopher and physician, widely regarded as one of the most influential of Enlightenment thinkers and commonly known as the "father of liberalism". Considered one of ...

's distinction between primary and secondary qualities. The theory that unobservables posited by scientific theories exist is referred to as

scientific realism Scientific realism is the view that the universe described by science is real regardless of how it may be interpreted. Within philosophy of science, this view is often an answer to the question "how is the success of science to be explained?" T ...

. It contrasts with

instrumentalism In philosophy of science and in epistemology, instrumentalism is a methodological view that ideas are useful instruments, and that the worth of an idea is based on how effective it is in explaining and predicting phenomena. According to instrumenta ...

, which asserts that we should withhold

ontological commitment An ontological commitment of a language is one or more objects postulated to exist by that language. The 'existence' referred to need not be 'real', but exist only in a universe of discourse. As an example, legal systems use vocabulary referring t ...

s 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

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

'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

. It contrasts with

, which asserts that we should withhold

s 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 In philosophy of science, constructive empiricism is a form of empiricism. While it is sometimes referred to as an empiricist form of structuralism, its main proponent, Bas van Fraassen, has consistently distinguished between the two views. Overvie ...

. According to

van Fraassen A van is a type of road vehicle used for transporting goods or people. Depending on the type of van, it can be bigger or smaller than a pickup truck and SUV, and bigger than a common car. There is some varying in the scope of the word across th ...

, 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