Deductive reasoning starts with a general hypothesis and builds a specific conclusion. It requires correct categorization to avoid wrong conclusions. A syllogism is a common form of deductive reasoning. Inductive reasoning works in the opposite order and can be disproven by counterexamples.
Deductive reasoning is one of the two basic forms of valid reasoning. Start with a general hypothesis or known fact and build a specific conclusion from that generalization. This is the opposite of inductive reasoning, which involves making broad generalizations from specific observations. The basic idea of deductive reasoning is that if something is true of a class of things in general, this truth applies to all members of that class. One of the keys to good deductive reasoning, then, is being able to correctly identify class members, because incorrect categorizations will lead to wrong conclusions.
Truth and Validity
For deductive reasoning to be valid, the original hypothesis or generalization must also be correct. A logical deduction can be made from any generalization, even if it is not true. If the generalization is wrong, however, the specific conclusion may be logical and valid, but it can still be wrong.
Examples
You can better understand deductive reasoning by looking at the examples. A generalization might be something like “All wasps have stingers.” The logical conclusion of a specific instance would therefore be: “That’s a wasp, so she has a sting.” This is a valid deduction. The truth of the deduction, however, depends on whether the observed insect is actually a wasp.
People often use deductive reasoning without even knowing it. For example, a parent might tell a child, “Watch out for that wasp—it might sting you.” The parent says this because he knows that wasps have stingers and, therefore, that the observed wasp has a sting and could sting the child.
Inductive reasoning
Inductive reasoning would work in the opposite order. The specific observation would be that a particular wasp has a stinger. It could therefore be assumed that all wasps have stingers. Many scientific tests involve proving whether a deduction or induction is, in fact, true. Inducing that all cats have orange fur because one cat has orange fur, for example, could easily be disproven by looking at cats that don’t have orange fur.
Syllogism
One of the most common and useful forms of deductive reasoning is the syllogism. A syllogism is a specific form of argument that has three simple steps: a major premise, a minor premise, and a logical conclusion. For example, the premise “Every X has characteristic Y” could be followed by the premise “This thing is X”, which would produce the conclusion “This thing has characteristic Y”. The first example of a wasp could be broken down into the major premise “Every wasp has a sting”, the minor premise “This insect is a wasp”, and the conclusion “This insect has a sting”. Creating a syllogism is considered a good way to test deductive reasoning to make sure it is valid.
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