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
atomic particles, the
force of gravity,
causation and
beliefs or
desires. However, some philosophers (ex.
George Berkeley) 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.