In informal logic, a reason consists of either a single premise or co-premises in support of an argument. In formal symbolic logic, only single premises occur. In informal reasoning, two types of reasons exist. An evidential reason is a foundation upon which to believe that or why a claim is true. An explanatory reason attempts to convince you how something is or could be true, but does not directly convince you that it is true.
In contrast to reason as an abstract noun, a reason is a consideration which explains or justifies.[1] Philosophers often distinguish explanatory reasons from normative or justifying reasons.[2]
Explanatory reasons are considerations which serve to explain why things have happened—they are reasons why 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 why 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 why 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, on the other hand, 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).[3][4] Some philosophers (one being John Broome[5]) view these 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).
A common philosopher's distinction concerning normative reasons is between epistemic reasons and practical reasons.[6] Epistemic reasons (also called theoretical 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).