unobservable entity

An unobservable (also called impalpable) is an entity whose existence, nature, properties, qualities or relations are not directly observable by humans. In philosophy of science, typical examples of "unobservables" are the force of gravity, causation and beliefs or desires. The distinction between observable and unobservable plays a central role in Immanuel Kant's distinction between noumena and phenomena as well as in John Locke's distinction between primary and secondary qualities. 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 Immanuel Kant's distinction between noumena and phenomena. 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.