- To apply reason is to calculate, to think; to offer in statement the justification for one’s intellectual position, or a rational motive and logical defense for a course of action. Reason is what makes fact intelligible.
- To reason is to engage in the applied effort of comprehending, inferring, and thinking in orderly, rational ways; to identify cause and effect. Reason is the power of the intellect by which man attains to truth or knowledge; the process of thinking rightly and justifiably.
- See also: Reasoning
Christianity and reason
For as God, when he created matter, and endued it with a principle of mobility, established certain rules for the perpetual direction of that motion; so, when he created man, and endued him with free-will to conduct himself in all parts of life, he laid down certain immutable laws of human nature, whereby that free-will is in some degree regulated and restrained, and gave him also the faculty of reason to discover the purport of those laws.
Christian apologetics is the defense of the Christian faith through logic/evidence based arguments. The term comes from the Greek word apologia, which means "defense". In addition, Christian apologist point out the falseness and deficiencies of opposing worldviews. John Gresham Machen declared, "False ideas are the greatest obstacles to the reception of the gospel".
Misconception of Reason
Too many people falsely believe that reason alone can lead to goodness, but reason can be used for good and for evil. As Dennis Prager puts it "Reason is just a tool."  As criminals use reasoning to commit a crime without their reason telling them what they are doing is wrong.  This misconception had its roots with the French Enlightenment with various philosophes such as Voltaire and Denis Diderot, and to a lesser extent Jean-Jacques Rousseau, where the emphasized humanity's advancement by "pure reason" alone, and also formed a large basis for the French Revolution and Reign of Terror.
- See Stanford Encyclopedia of Philosophy: Kant's Account of Reason Consistency in observations is generally sufficient to confirm everyday knowledge claims: "the law of reason to seek unity is necessary, since without it we would have no reason, and without that, no coherent use of the understanding, and, lacking that, no sufficient mark of empirical truth…"
See also Catholic Encyclopedia: Reason. Man is rational in the sense that he is a being who arrives at conclusions from premises. Our intellectual life is a process, a voyage of discovery; our knowledge is not a static ready-made whole; it is rather an organism instinct with life and growth. Each new conclusion becomes the basis of further inference.
- The War on Reason
- The Reasonableness of Christianity, as delivered in the Scriptures, by John Locke
- Commentaries on the Laws of England, Volume 1, p. 26