A reason is a consideration which justifies or explains.
In moral philosophy, it is common to distinguish between normative reasons and explanatory reasons.
Explanatory reasons are considerations which serve to explain why things have happened—they are reasons events occur, or why states of affairs are the way they are. In other words, "reason" can also be a synonym for "cause". For example, a reason a car starts is that its ignition is turned. In the context of explaining the actions of beings who act for reasons (i.e., rational agents), these are called motivating reasons—e.g., the reason Bill went to college was to learn; i.e., that he would learn was his motivating reason. At least where a rational agent is acting rationally, her motivating reasons are those considerations which she believes count in favor of her so acting.
Normative reasons (i.e. justifying reasons) are often said to be "considerations which count in favor" of some state of affairs (this is, at any rate, a common view, notably held by T. M. Scanlon and Derek Parfit).
Epistemic vs. practical reasons
Philosophers, when discussing reasoning that is influenced by norms, commonly make a distinction between theoretical reason and practical reason. These are capacities that draw on epistemic reasons (matters of fact and of explanation) or practical reasons (reasons for action) respectively. Epistemic reasons (also called theoretical or evidential reasons) are considerations which count in favor of believing some proposition to be true. Practical reasons are considerations which count in favor of some action or the having of some attitude (or at least, count in favor of wanting or trying to bring those actions or attitudes about).
Some philosophers (one being John Broome) view normative reasons as the same as "explanations of ought facts". Just as explanatory reasons explain why some descriptive fact obtains (or came to obtain), normative reasons on this view explain why some normative facts obtain, i.e., they explain why some state of affairs ought to come to obtain (e.g., why someone should act or why some event ought to take place).
Internalism and Externalism about Reasons
The 'internalist' or 'Humean' account of reasons is most famously associated with Bernard Williams. Roughly, the internalist view is that all reasons are connected to or provided by desires. Very roughly, you have reason to do something only if it is something you want (or will get you something you want).
This is rough in two respects. First, philosophers work with a very broad notion of 'desire'. Second, to retain the normativity of reasons, there is room for mistakes. We are often mistaken about what we desire, or about what would be the best route to satisfying those desires.
Put in this bare form, Humeanism appears to have some quite counterintuitive consequences: in an extreme example, someone who has no desires that would be furthered by saving the life of a child has no reason to do so.
'Externalists' say that there are (perhaps also) many reasons which one has regardless of one's desires.
- Merriam-Webster.com Merriam-Webster Dictionary definition of reason
- Scanlon, T.M. (2000-11-15). What We Owe to Each Other. Belknap Press. p. 17. ISBN 9780674004238.
- Parfit, Derek (January 23, 2009). On What Matters (forthcoming) (PDF). Rutgers University. Archived from the original (PDF) on March 31, 2010. Retrieved September 16, 2011.
- Wallace, R. Jay (2014-01-01). Zalta, Edward N., ed. Practical Reason (Summer 2014 ed.).
- Broome, John. "Reasons". In Reason and Value: Themes from the Moral Philosophy of Joseph Raz. Edited by R. Jay Wallace et al. Oxford: Oxford University Press, 2004. p. 28.