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.