# Unknowability

In philosophy, unknowability is the possibility of inherently unaccessible knowledge. It addresses the epistemology of that which cannot be known. Some related concepts include the limits of knowledge, ignorabimus, unknown unknowns, the halting problem, and chaos theory.
Nicholas Rescher provides the most recent focused scholarship for this area in Unknowability: An Inquiry into the Limits of Knowledge, where he offered three high level categories, logical unknowability, conceptual unknowability, and in-principle unknowability.

## Background

Speculation about what is knowable and unknowable has been part of the philosophical tradition since the inception of philosophy. In particular, Baruch Spinoza's Theory of Attributes argues that a human's finite mind cannot understand infinite substance; accordingly, infinite substance, as it is in itself, is in-principle unknowable to the finite mind.
Immanuel Kant brought focus to unknowability theory in his use of the noumenon concept. He postulated that, while we can know the noumenal exists, it is not itself sensible and must therefore remain unknowable.
Modern inquiry encompasses undecidable problems and questions such as the halting problem, which in their very nature cannot be possibly answered. This area of study has a long and somewhat diffuse history as the challenge arises in many areas of scholarly and practical investigations.

## Rescher's categories of unknowability

Rescher organizes unknowability in three major categories:
* logical unknowability — arising from abstract considerations of epistemic logic.
* conceptual unknowability — analytically demonstrable of unknowability based on concepts and involved.
* in-principle unknowability — based on fundamental principles.
In-principle unknowability may also be due to a need for more energy and matter than is available in the universe to answer a question, or due to fundamental reasons associated with the quantum nature of matter. In the physics of special and general relativity, the light cone marks the boundary of physically knowable events.

## The halting problem and the Diophantine problem

The halting problem – namely, the problem of determining if arbitrary computer programs will ever finish running – is a prominent example of an unknowability associated with the established mathematical field of computability theory. In 1936, Alan Turing proved that the halting problem is undecidable. This means that there is no algorithm that can take as input a program and determine whether it will halt. In 1970, Yuri Matiyasevich proved that the Diophantine problem (closely related to Hilbert's tenth problem) is also undecidable by reducing it to the halting problem. This means that there is no algorithm that can take as input a Diophantine equation and always determine whether it has a solution in integers.
The undecidability of the halting problem and the Diophantine problem has a number of implications for mathematics and computer science. For example, it means that there is no general algorithm for proving that a given mathematical statement is true or false. It also means that there is no general algorithm for finding solutions to Diophantine equations.
In principle, many problems can be reduced to the halting problem. See the list of undecidable problems.
Gödel's incompleteness theorems demonstrate the implicit in-principle unknowability of methods to prove consistency and completeness of foundation mathematical systems.

## Related concepts

There are various graduations of unknowability associated with frameworks of discussion. For example:
* unknowability to particular individual humans (due to individual limitations);
* unknowability to humans at a particular time (due to lack of appropriate tools);
* unknowability to humans due to limits of matter and energy in the universe that might be required to conduct the appropriate experiments or conduct the calculations required;
* unknowability to any processes, organism, or artifact.
Treatment of knowledge has been wide and diverse. Wikipedia itself is an initiate to capture and record knowledge using contemporary technological tools. Earlier attempts to capture and record knowledge include writing deep tracts on specific topics as well as the use of encyclopedias to organize and summarize entire fields or event the entirety of human knowledge.

### Limits of knowledge

An associated topic that comes up frequently is that of Limits of Knowledge.
Examples of scholarly discussions involving limits of knowledge include:
* John Horgan's End of science: facing the limits of knowledge in the twilight of the scientific age.
* Tavel Morton's Contemporary physics and the limits of knowledge.
* Christopher Cherniak's Limits for knowledge.
* Ignoramus et ignorabimus, a Latin maxim meaning "we do not know and will not know", popularized by Emil du Bois-Reymond. Bois-Reymond's ignorabimus proclamation was viewed by David Hilbert as unsatisfactory, and motivated Hilbert to declare in 1900 International Congress of Mathematicians that answers to problems of mathematics are possible with human effort. He declared, "in mathematics there is no ignorabimus",. The halting problem and the Diophantine Problem eventually were answered demonstrating in-principle unknowability of answers to some foundational mathematical questions, meaning Bois-Reymond's assertion was in fact correct.
Gregory Chaitin discusses unknowability in many of his works.

### Categories of unknowns

Popular discussion of unknowability grew with the use of the phrase There are unknown unknowns by United States Secretary of Defense Donald Rumsfeld at a news briefing on February 12, 2002. In addition to unknown unknowns there are known unknowns and unknown knowns. These category labels appeared in discussion of identification of chemical substances.

### Chaos theory

Chaos theory is a theory of dynamics that argues that, for sufficiently complex systems, even if we know initial conditions fairly well, measurement errors and computational limitations render fully correct long-term prediction impossible, hence guaranteeing ultimate unknowability of physical system behaviors.