Short answers, pulled from the story.
Reason is the capacity to consciously apply logic by drawing valid conclusions from new or existing information, with the aim of seeking truth. It is associated with activities considered characteristic of humans, including philosophy, religion, science, language, and mathematics.
The English word reason comes from the French raison, which descends directly from the Latin ratio. Ratio was itself a translation of the Greek logos, a term that could mean speech, explanation, or an account of money handled.
Aristotle defined human beings as rational animals, emphasizing reason as a characteristic of human nature. He described the highest human happiness, called eudaimonia, as a life lived consistently and excellently in accordance with reason.
David Hume remarked that reason is, and ought only to be, the slave of the passions, and can never pretend to any office other than to serve and obey them. He also argued that cause and effect cannot be deduced, so no knowledge rests on reasoning alone.
Immanuel Kant's categorical imperative states: act only according to that maxim whereby you can, at the same time, will that it should become a universal law. It justifies an action only if the underlying rule could be universalized.
Reasoning is commonly subdivided into deductive reasoning, inductive reasoning, abductive reasoning, and analogical reasoning. In deduction a conclusion follows necessarily from the premises, while in induction the conclusion follows only with some degree of probability.
Jean-Jacques Rousseau proposed in his second Discourse that reason and political life are not natural and may even be harmful to mankind. He argued that reason, language, and rational community developed gradually because certain habits of cooperation solved particular problems.