The authors of model-dependent realism claim that what type of theory may not exist?
Model-dependent realism Model-dependent realism is a view of scientific inquiry that focuses on the role of scientific models of phenomena. It claims reality should be interpreted based upon these models, and where several models overlap in describing a particular subject, multiple, equally valid, realities exist. It claims that it is meaningless to talk about the "true reality" of a model as we can never be "absolutely" certain of anything. The only meaningful thing is the usefulness of the model. The term "model-dependent realism" was coined by Stephen Hawking and Leonard Mlodinow in their 2010 book, "The Grand Design".
Noumenon In metaphysics, the noumenon ( , , from Greek: [εν]νοούμενον) is a posited object or event that exists without sense or perception. The term "noumenon" is generally used in contrast with or in relation to "phenomenon", which refers to anything that can be apprehended by or is an object of the senses. Modern philosophy has generally been skeptical of the possibility of knowledge independent of the senses, and Immanuel Kant gave this point of view its canonical expression: that the noumenal world may exist, but it is completely unknowable through human sensation. In Kantian philosophy, the unknowable noumenon is often linked to the unknowable "thing-in-itself" (in Kant's German, "Ding an sich"), although how to characterize the nature of the relationship is a question yet open to some controversy.