Affirming a disjunct

Affirming a disjunct occurs in a deductive argument when it is assumed, because one of multiple possibilities is true, that the other or others are false. It is a misuse use of the However, in select circumstances under the right conditions this is not a fallacy, but a logically valid argument when the law of excluded middle is correctly applied.
It is a syllogistic fallacy and a formal fallacy.
==Alternate names==
This one's got an endless stream of boring alternate titles:
*(Fallacy of) Affirming a/the/one disjunct
*(Fallacy of) Affirmation of a/the/one disjunct
*(Fallacy of) Alternative syllogism
*(Fallacy of) Alternative disjunct
*(Fallacy of) Asserting an alternative
*(Fallacy of) A/the/one disjunctive syllogism
*(Fallacy of) Improper disjunctive syllogism
*(Fallacy of) False exclusionary disjunct
==Form==
Affirming a disjunct can take two forms:
Accepting the first term and denying the second:
Accepting the second term and denying the first:
=== Symbolic form ===
==Error==
Affirming a disjunct is fallacious because both options can be true at the same time, making the conclusion invalid.
==Examples==
===Is the bird alive?===
Both disjuncts can be true - a bird can be both alive and on the ground.
===Who died on the fourth of July, 1826?===
Thomas Jefferson!
John Adams!
===Is Max a mammal?===
The problem here is that "or" is in an inclusive sense, not an exclusive sense. A cat is in fact a mammal.
===Fact or theory?===
===Who's on the cover?===
Again, "or" is in an inclusive sense.
==Legitimate use==
The only reason this fallacy occurs is because of the lack of clarity in the term "or" and the logical structure of the argument.
===Different "or"===
You only fallaciously affirm a disjunct when something is not a dilemma — when both options can be true — but you assert that only one can be true. If something truly is a dilemma, then affirming a disjunct is not fallacious. For example:
This is a valid conclusion because the "or" is exclusive — only one may be true at a time.
This is a valid conclusion because the situation is a binary one — there are only two options, of which neither can be true while the other is simultaneously true.
"Or" usually has two meanings in logic:
*Inclusive (or "weak") disjunction (A or B): Implies A, or B, or both. One or both of the disjuncts is true, which is what is meant by the "and/or" of legalese. Affirming a Disjunct is a non-validating form of argument when "or" is inclusive, as it is usually interpreted in propositional logic. 
*Exclusive (or "strong") disjunction (A xor B): Implies A, or B, but not both. Exactly one of the disjuncts is true.
===Enthymeme===
Alternately, one can view this fallacy as that of a hidden or suppressed premise.
Consider this reformulation of the first form of affirming a disjunct:
This statement is logically valid.
If we have reason to think that such a hidden premise is true (perhaps through tone of voice, outside knowledge, etc.) then we might assume that an exclusionary premise P1.5 exists. If, alternately, it is possible for both A and B to be true, it should be assumed that no such hidden premise exists.
==External links==
*Affirming a Disjunct — Logically Fallacious
*Affirming a disjunct — Philosophy Index
*Affirming a disjunct — LogFall
*Affirming the Disjunct — How to Never Be Wrong Again 
==References==
Category:Formal fallacies