Empiricists only study observable behaviour instead of unobservable mental representations, states and processes. In field theories, different configurations of the unobservable fields can result in identical observable quantities. Instrumentalism exhibited by "scientists" often does not even discern unobservable from observable entities. This horizon represents the boundary between the observable and the unobservable regions of the universe. Qualia are unobservable in others and unquantifiable in us. These distributions may apply to unobservable events or conditional probabilities. Structural equation models are often used to assess unobservable 'latent' constructs. Thus, the McCallum rule can overcome the problem of the unobservable variables. Hume, however, held that such unobservable metaphysical concepts should be rejected as "sophistry and illusion". One is the logically unobservable, which involves a contradiction. This undermines the case for even a transient, unobservable existence for such virtual particles. A system is detectable if all the unobservable states are stable. Solutions to problems of an uncontrollable or unobservable system include adding actuators and sensors. Soon, most physicists accepted that atoms and molecules were unobservable yet real. According to Marx himself, the "essence" is not some kind of esoteric, mystical "unobservable" kernel.