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For example, the proposition that water is H 2 O (if it is true): According to Kripke, this statement is both necessarily true, because water and H 2 O are the same thing, they are identical in every possible world, and truths of identity are logically necessary; and a posteriori, because it is known only through empirical investigation.
Since assuming P to be false leads to a contradiction, it is concluded that P is in fact true. An important special case is the existence proof by contradiction: in order to demonstrate that an object with a given property exists, we derive a contradiction from the assumption that all objects satisfy the negation of the property.
[88] (opposite of appeal to tradition) Appeal to poverty (argumentum ad Lazarum) – supporting a conclusion because the arguer is poor (or refuting because the arguer is wealthy). (Opposite of appeal to wealth.) [89] Appeal to tradition (argumentum ad antiquitatem) – a conclusion supported solely because it has long been held to be true. [90]
A system will be said to be inconsistent if it yields the assertion of the unmodified variable p [S in the Newman and Nagel examples]. In other words, the notion of "contradiction" can be dispensed when constructing a proof of consistency; what replaces it is the notion of "mutually exclusive and exhaustive" classes.
For example, "I've flipped heads with this coin five times consecutively, so the chance of tails coming out on the sixth flip is much greater than heads." [ 66 ] Hot-hand fallacy (also known as "hot hand phenomenon" or "hot hand"), the belief that a person who has experienced success with a random event has a greater chance of further success ...
A statement is logically true if, and only if its opposite is logically false. The opposite statements must contradict one another. In this way all logical connectives can be expressed in terms of preserving logical truth. The logical form of a sentence is determined by its semantic or syntactic structure and by the placement of logical constants.
The fact–value distinction is also closely related to the moralistic fallacy, an invalid inference of factual conclusions from purely evaluative premises. For example, an invalid inference "Because everybody ought to be equal, there are no innate genetic differences between people" is an instance of the moralistic fallacy.
Reductio ad absurdum, painting by John Pettie exhibited at the Royal Academy in 1884. In logic, reductio ad absurdum (Latin for "reduction to absurdity"), also known as argumentum ad absurdum (Latin for "argument to absurdity") or apagogical arguments, is the form of argument that attempts to establish a claim by showing that the opposite scenario would lead to absurdity or contradiction.