Counterfactual definiteness

Because of the importance of the observer effect and other factors, counterfactual definiteness (CFD) is an open question in quantum mechanics. Whether experts working in the field can speak "meaningfully" of the definiteness of the results of measurements that have not been performed is not known for certain. Theorists often assume definitely that various objects exist and that objects have certain properties, but under a counterfactual scenario (i.e., when no measurement is made), the scientific principles at work don't apply as cleanly, and such assumptions are not as solid as under rigorous experimental conditions. The term "counterfactual definiteness" is used in discussions of physics calculations, especially those related to the phenomenon called quantum entanglement and those related to the Bell inequalities. In such discussions "meaningfully" means the ability to treat these unmeasured results on an equal footing with measured results in statistical calculations. It is this (sometimes assumed but unstated) aspect of counterfactual definiteness that is of direct relevance to physics and mathematical models of physical systems and not philosophical concerns regarding the meaning of unmeasured results.