Deductive reasoning is one of the two basic forms of valid reasoning. It begins with a general hypothesis or known fact and creates a specific conclusion from that generalization. This is the opposite of inductive reasoning, which involves creating 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 for sound deductive reasoning, then, is to be able to properly identify members of the class, because incorrect categorizations will result in unsound conclusions.
Truth and Validity
For deductive reasoning to be sound, the original hypothesis or generalization also must be correct. A logical deduction can be made from any generalization, even if it is not true. If the generalization is wrong, though, the specific conclusion can be logical and valid but still can be incorrect.
One can better understand deductive reasoning by looking at examples. A generalization might be something such as, "All wasps have stingers." The logical conclusion of a specific instance would then be, "That is a wasp, so it has a stinger." This is a valid deduction. The truth of the deduction, however, depends on whether the observed insect is, indeed, a wasp.
People often use deductive reasoning without even knowing it. For example, a parent might say to a child, "Be careful of that wasp — it might sting you." The parent says this because he or she knows that wasps have stingers and, therefore, that the observed wasp has a stinger and might sting the child.
Inductive reasoning would work in the opposite order. The specific observation would be that a particular wasp has a stinger. One could then induce 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 be easily disproved by observing cats that do not have orange fur.
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 easy steps: a major premise, a minor premise and a logical conclusion. For example, the premise "Every X has the characteristic Y" could be followed by the premise "This thing is X," which would yield the conclusion "This thing has the characteristic Y." The first wasp example could be broken up into the major premise "Every wasp has a stinger," the minor premise "This insect is a wasp" and the conclusion "This insect has a stinger." Creating a syllogism is considered a good way for deductive reasoning to be tested to ensure that it is valid.