Deductive logic starts with true premises and deduces a logical conclusion, while inductive logic explains the reason behind known facts. Deductive arguments can be valid but not true, and invalid arguments occur when the conclusion doesn’t follow logically from the premises. Both deductive and inductive logic are used in arguments only.
Most often used in philosophy, deductive logic is one of two classifications for logical arguments. Unlike inductive logic, deductive logic starts with supposedly true premises and then makes a deduction from those premises. Both deductive and inductive logic are used in arguments only.
Inductive logic begins with facts known to be true, such as statistics, and aims to explain the reason behind those statistics in a logical and reasonable way. Deductive logic, on the other hand, starts from a set of premises and deduces a logical conclusion from these premises. Premises are statements that may or may not be true, but are taken as fact for purposes of the argument.
Deductive logic deals with the structure of the argument more than with the content of the argument. In a deductive argument, it is stated that premise A and premise B are true, and therefore conclusion C is also true. Outside of philosophy, geometric proofs are a type of deductive logic. In fact, the structure can be clearly seen using simple numbers. For example, assuming that A equals 1 and B equals 2, then C must equal 3.
A deductive argument can be valid or invalid. Since deductive arguments are based on the assumption that the premises are true, an argument can be valid without being true. For example, the topic: “All gray-haired women are grandmothers. Betty is a gray haired woman. So Betty must be a grandmother,” it is valid but it is false.
If the premises are true, the conclusion that Betty is a grandmother is also true. The first premise, however, is not true. All gray-haired women aren’t necessarily grandmothers, so Betty isn’t necessarily a grandmother. If an argument is valid but not true, then a premise in that argument is false.
Invalid arguments occur when the conclusion does not follow logically from the premises. The following is an example of an invalid argument: “Layla is always hungry after dancing. Layla is hungry. Therefore, Layla danced.”
Dancing is just one of the many things that cause hunger. Layla might be hungry because she danced, but she might be hungry simply because she hasn’t eaten all day. Although the premises are true, the conclusion does not follow logically from the premises and, therefore, is invalid.
Both deductive and inductive logic are used in arguments only. Arguments are things that are debatable or that one can disagree about. Descriptions or mere opinion are not arguments and, therefore, no form of logic can be used. For example, saying, “My favorite ice cream is butter pecan” is just an opinion and therefore cannot be disputed.
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