An unobservable 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. The theory that unobservables posited by scientific theories exist is referred to as scientific realism. 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. 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.