Epistemic possibility
Encyclopedia
In philosophy
and modal logic
, epistemic possibility relates a statement under consideration to the current state of our knowledge
about the actual world: a statement is said to be:
Epistemic possibility is often contrasted with subjunctive possibility
(or alethic possibility), and although epistemic and subjunctive possibilities are often expressed using the same modal terms (such as possibly, could be, must be) or similar modal terms that are sometimes confused (such as may be and might be), statements that are qualified in terms of epistemic possibility and statements that are qualified in terms of subjunctive possibility have importantly different meanings.
The contrast is best explained by example. Consider the two statements:
Although these two statements are often confused with one another, they mean two different things: the first says something true about the vagaries of war; the second says something that is certainly false. The difference comes from the fact that the first statement—a statement of subjunctive possibility—says something about how things might have been under counterfactual
conditions, whereas the second—a statement of epistemic possibility—says something about the relation between a particular outcome (a victory by Hitler) and our knowledge about the actual world (since, as it happens, we know perfectly well that that particular outcome did not actually obtain, we know that what it says is false).
The parallel distinction arises between types of conditionals (if-then statements). Consider the difference between the epistemic connection expressed by an indicative conditional and the causal or metaphysical relation expressed by a subjunctive conditional:
The first statement says something that is certainly true, and will be accepted as such by anyone who is convinced that somebody shot Kennedy. It's clearly true because it expresses this epistemic relation between its antecedent (the "if"-clause) and its consequent (the "then"-clause): The antecedent, if we came to know it was true, would provide us with excellent evidence that the consequent is true. The second statement, on the other hand, expresses a causal or metaphysical relation: It says that the world was set up so that the consequent would have been made true if the antecedent were true. One will accept that second statement to the extent that one thinks the world was set up in that way. (Conspiracy theorists who think there was a back-up shooter, for instance, may accept the second statement.)
Because of these differences, epistemic possibility bears on the actual world in ways that subjunctive possibility does not. Suppose, for example, that I want to know whether or not to take an umbrella before I leave. If you tell me "It's possible that it is raining outside"—in the sense of epistemic possibility—then that would weigh on whether or not I take the umbrella. But if you just tell me that "It's possible for it to rain outside"—in the sense of metaphysical possibility—then I am no better off for this bit of modal enlightenment.
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...
and modal logic
Modal logic
Modal logic is a type of formal logic that extends classical propositional and predicate logic to include operators expressing modality. Modals — words that express modalities — qualify a statement. For example, the statement "John is happy" might be qualified by saying that John is...
, epistemic possibility relates a statement under consideration to the current state of our knowledge
Knowledge
Knowledge is a familiarity with someone or something unknown, which can include information, facts, descriptions, or skills acquired through experience or education. It can refer to the theoretical or practical understanding of a subject...
about the actual world: a statement is said to be:
- epistemically possible if it may be true, for all we know
- epistemically necessary if it is certain (or must be the case, given what we know)
- epistemically impossible if it cannot be true, given what we know
Epistemic possibility is often contrasted with subjunctive possibility
Subjunctive possibility
Subjunctive possibility is the form of modality most frequently studied in modal logic...
(or alethic possibility), and although epistemic and subjunctive possibilities are often expressed using the same modal terms (such as possibly, could be, must be) or similar modal terms that are sometimes confused (such as may be and might be), statements that are qualified in terms of epistemic possibility and statements that are qualified in terms of subjunctive possibility have importantly different meanings.
The contrast is best explained by example. Consider the two statements:
- Hitler might have been victorious in World War IIWorld War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
- Hitler may have been victorious in World War IIWorld War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
Although these two statements are often confused with one another, they mean two different things: the first says something true about the vagaries of war; the second says something that is certainly false. The difference comes from the fact that the first statement—a statement of subjunctive possibility—says something about how things might have been under counterfactual
Counterfactual
Counterfactual may refer to:* Counterfactual conditional, a grammatical form * Counterfactual subjunctive, grammatical forms which in English are known as the past and pluperfect forms of the subjunctive mood* Counterfactual thinking* Counterfactual history* Alternate history, a literary genre*...
conditions, whereas the second—a statement of epistemic possibility—says something about the relation between a particular outcome (a victory by Hitler) and our knowledge about the actual world (since, as it happens, we know perfectly well that that particular outcome did not actually obtain, we know that what it says is false).
The parallel distinction arises between types of conditionals (if-then statements). Consider the difference between the epistemic connection expressed by an indicative conditional and the causal or metaphysical relation expressed by a subjunctive conditional:
- If OswaldLee Harvey OswaldLee Harvey Oswald was, according to four government investigations,These were investigations by: the Federal Bureau of Investigation , the Warren Commission , the House Select Committee on Assassinations , and the Dallas Police Department. the sniper who assassinated John F...
didn't shoot KennedyJohn F. KennedyJohn Fitzgerald "Jack" Kennedy , often referred to by his initials JFK, was the 35th President of the United States, serving from 1961 until his assassination in 1963....
, someone else did - If OswaldLee Harvey OswaldLee Harvey Oswald was, according to four government investigations,These were investigations by: the Federal Bureau of Investigation , the Warren Commission , the House Select Committee on Assassinations , and the Dallas Police Department. the sniper who assassinated John F...
hadn't shot KennedyJohn F. KennedyJohn Fitzgerald "Jack" Kennedy , often referred to by his initials JFK, was the 35th President of the United States, serving from 1961 until his assassination in 1963....
, someone else would have
The first statement says something that is certainly true, and will be accepted as such by anyone who is convinced that somebody shot Kennedy. It's clearly true because it expresses this epistemic relation between its antecedent (the "if"-clause) and its consequent (the "then"-clause): The antecedent, if we came to know it was true, would provide us with excellent evidence that the consequent is true. The second statement, on the other hand, expresses a causal or metaphysical relation: It says that the world was set up so that the consequent would have been made true if the antecedent were true. One will accept that second statement to the extent that one thinks the world was set up in that way. (Conspiracy theorists who think there was a back-up shooter, for instance, may accept the second statement.)
Because of these differences, epistemic possibility bears on the actual world in ways that subjunctive possibility does not. Suppose, for example, that I want to know whether or not to take an umbrella before I leave. If you tell me "It's possible that it is raining outside"—in the sense of epistemic possibility—then that would weigh on whether or not I take the umbrella. But if you just tell me that "It's possible for it to rain outside"—in the sense of metaphysical possibility—then I am no better off for this bit of modal enlightenment.
Further reading
- Alan R. White, Modal Thinking (ISBN 0-631-16560-6), chapter 1. White discusses subjunctive possibility as "the actuality of a possibility" and epistemic possibility as "the possibility of an actuality"
- Keith DeRose, "Epistemic Possibilities," The Philosophical Review 100 (1991): pp. 581–605.
- David Chalmers, The Components of Content and The Tyranny of the Subjunctive