We actually mean a lot of different things when we refer to reason. It is the way in which we try to make sense of the world using logic, rationality, comparison, judgement, and experience, although it is another word that is used in a lot of different ways. The OED gives us…
1 a cause, explanation, or justification. 2 good or obvious cause to do something: we have reason to celebrate. 3 the power of the mind to think, understand, and form judgements logically. 4 (one’s reason) one’s sanity. 5 what is right, practical, or possible: I’ll answer anything, within reason. 6. think, understand, and form judgements logically. 7 (reason out) find a solution (to a problem) by considering possible options. 8 (reason with) persuade with rational argument. — PHRASES by reason of formal because of. listen to reason be persuaded to act sensibly. it stands to reason it is obvious or logical.
Reason is something that we use whenever we make a decision, and most of the time, our reasoning occurs instinctively, as we decide on the best path to take almost unconsciously, depending on previous experiences involving similar situations. It is possible to train ourselves to reason consciously, though, and the more one thinks about the decisions one is making, the more one is able to have active control over them. Some people have termed this intellectual enlightenment, the point where we are consciously aware of every choice we ever make. The extent to which this is achieved determines how rational we are.
Questions that need to be considered include how we reason, its relationship with emotion, the extent to which we can trust decisions that have been made using reason, and the fallacies that can arise when reason is not used properly.