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.
Noumena are the things-in-themselves, i.e., raw things in their necessarily unknowable state,[3] 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".
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.
[5] Some philosophers have been motivated by these and similar examples to question the value of the distinction between observable and unobservable in general.