Rescher demonstrates, from within the interstices of traditional Logic, the essential class of unknowable truths to be a necessary feature borne out of finite cognitive abilities. His classification of unknowabilities according to their origin in evolutionary impredictability, verificational surdity, ontological excess, and vagrant predicates is built into a robust formal logical, and multi-value semantic-pragmatic, account of the individuated unspecifiability, unexemplifiability, and noncountability that determine the outermost reaches of reason.
Notable for its general sobriety, this towering work of philosophy cuts down science's claims to explanatory ultimacy while simultaneously appealing to the inability of present science to rule out other more efficient modalities of explanation presently considered "unscientific", e.g. teleology for accounts of causal relations between indiscernables, that will inevitably expand the very domain of science due to newer conceptual possibilities that emerge in the wake of still unforeseeable developments.
This is a book that delivers the goods that all "theory of everything" books routinely fail to. Agnoseology, the study of the epistemic-ontological rift between propositionality and experience, or the limits of knowledge and the facts of reality, as a project certainly goes a long way to rehabilitate the status of metaphysical problems as actual problems that demand logico-philosophical solutions.