What Is Reason?

Jack Krupansky
4 min readMar 31, 2018


What is reasoning (or reason)? This informal paper presents a relatively detailed definition of the term. Not all of the gory details of all forms of reason, but a reasonably precise definition. A working definition that should cover most situations. Including artificial intelligence, especially Strong AI.

The definitions of reason and reasoning presented here are based on the definitions in a larger companion paper, Vocabulary of Knowledge, Thought, and Reason. The point of this separate paper is to make those definitions more accessible than being buried in a discourse on knowledge.

The core term is reasoning, which is the process of engaging in reason, the process of reaching a conclusion, understanding, decision, or other end goal using rational intellectual thought.

Reason is more of a very high level abstraction for the overall reasoning process.

Now on to the detailed definitions of reason and reasoning.

Definition of reason

Reason can be defined as:

  • Abstract process of reasoning through rational thought, to reach a conclusion, result, goal, decision, judgment, assessment, understanding, or other outcome that is thoroughly and convincingly justified by the reasoning process.
  • Alternatively, a proposition which provides specific support for an argument, conclusion, or explanation for a fact.
  • Alternatively, a credible explanation, ground, or motive for an action or belief, as opposed to a mere excuse which may be based on nothing more than emotion.

This definition is a bit vague and abstract, which is a consequence of the real meat of its meaning being covered by the definition of reasoning.

Definition of reasoning

Reasoning can be defined as:

  • Rational intellectual process of arriving at a conclusion, result, goal, decision, judgment, assessment, understanding, or other outcome that is thoroughly and convincingly justified through a rational thought process grounded in facts, competent and credible analysis, and sound judgment which involves intentional, considered, thoughtful, orderly, credible, sensible, and otherwise rational steps or arguments, such as argumentation or logic.
  • Generally dispassionate, but may be informed, influenced, shaped, or constrained or otherwise limited by emotional, social, or practical considerations, as well as shortcuts and leaps such as heuristics and intuition.
  • Yes, intuition can count for reasoning, provided it is based on and informed by some significant degree of experience and judgment.
  • May also be based on common sense and general wisdom.
  • The point or purpose of reasoning is to achieve an optimal, best, fair, just, and persuasive outcome.
  • The process may start with observations, evidence, facts, assumptions, and possibly a desired or intended goal or proposed conclusion.
  • The process may proceed wherever the evidence leads it, or may proceed by driving towards a desired result.
  • May be informal or formal, weak or strong.
  • May use informal argument, casual or rhetorical argumentation, or tight, formal logic, or even rigorous mathematical proof.
  • By default, reasoning is presumed to be fairly rigorous — strong reasoning, but in practice tends to be somewhat weaker than rigorous logic.
  • There is no universal, purely objective form of reasoning. Reasoning may be subjective and may be relative to the values of the individual or group engaged in reasoning.
  • Assumptions and even logic itself may be influenced, constrained, or driven by values and other subjective considerations.
  • A conclusion reached through reasoning is considered knowledge.The steps of the process are in themselves knowledge.
  • Whether the reasoning behind a conclusion should be considered as part of the conclusion or distinct from the conclusion is not so clear, debatable, and may depend on the context in which the conclusion is used.
  • See also: argumentation.
  • See also: deductive reasoning, inductive reasoning, scientific reasoning, rationale, and foregone conclusion.
  • See also: sound judgment, reasonable.

What’s reasonable?

A definition for reasonable:

  • A belief, conclusion, decision, or action which at least superficially seems in concordance with sound judgment, fairness, and sensibility.
  • May or may not be based on strong reasoning.
  • May not even be based on weak reasoning or any reasoning at all, but simply be considered acceptable and not in conflict with sound judgment.

How formal does reasoning have to be?

As noted in the definition used here, reasoning can range across a very wide spectrum, from very informal to very formal.

Strict, formal logic and mathematical perfection is not required in general, although they may be appropriate or even required in some situations.

Weak reasoning and strong reasoning are both still reasoning.

Sure, there is and should be a very clear bias in favor of strong reasoning, but time and available information or the needs of the particular situation may argue against or interfere with stronger reasoning.

Definitions of other terms related to reason

A number of other terms related to reason and reasoning are defined in that other paper, Vocabulary of Knowledge, Thought, and Reason, including:

  • Argument
  • Argumentation
  • Belief
  • Common sense
  • Conclusion
  • Deductive reasoning
  • Fact
  • Foregone conclusion
  • General wisdom
  • Inductive reasoning
  • Intelligence
  • Intuition
  • Knowledge
  • Logic
  • Predicate
  • Proposition
  • Rational
  • Rationale
  • Reasonable
  • Scientific reasoning
  • Sound judgment
  • Strong reasoning
  • Weak reasoning

Artificial intelligence (AI)

More on reasoning as applied to artificial intelligence, particularly so-called Strong AI, can be found in a companion paper, Untangling the Definitions of Artificial Intelligence, Machine Intelligence, and Machine Learning.


Ditto for more on reason in the larger context of intelligence. See that same paper, Untangling the Definitions of Artificial Intelligence, Machine Intelligence, and Machine Learning.


Reason is a rather abstract concept.

Reasoning can be a very complex process.

There is no single, generalized, narrow definition of reasoning which adequately covers all situations.

Reasoning spans a very broad spectrum.

Reasoning can be very simple. Or very complex.

Reasoning can be very formal. Or very informal.

Reasoning can be very strong. Or very weak.

Reasoning should be tailored to the needs of the situation in which it is used.


  1. Vocabulary of Knowledge, Thought, and Reason.
  2. Untangling the Definitions of Artificial Intelligence, Machine Intelligence, and Machine Learning.

For more of my writings on artificial intelligence, see List of My Artificial Intelligence (AI) Papers.