Deduction is one of the three basic types of reasoning. Deductive arguments are those in which the conclusion follows from the premisses solely because of the agreed meaning of the syncategorematic words used to express the argment. Syncategorematic words are connector words, such as 'if', 'and', 'or', 'not', 'all', 'are', etc. The term "syncategorimatic" is a medieval term meaning "with category words." The point is that syncategorematic words do not, by themselves, make any reference to objects; they merely structure propositions (and arguments). They have meaning only if they occur with content-bearing words, called categorematic words. For example, if we let the letters A, B, and C represent categorematic words--the terms of an argument--then any argument of the form, "All A are B and all B are C, so all A are C," is a deductive argument. This argument turns primarily on the meaning of the words "all" and "are," which express the concept of class inclusion, but the meanings of "and" and "so" are also involved.

Because deductive arguments turn upon the meaning of words, the conclusion of a valid deductive argument follows from the premisses by definition. The premisses, in effect, stipulate that the conclusion is true. For this reason, there can be no doubt about the conclusion, given the truth of the premisses: if the premisses are true, the conclusion must be true. For this reason deduction is rightly considered the strongest type of reasoning.

On the other hand, while deduction is the strongest type of reasoning, it is also the least useful in terms of giving us new information. It tells us nothing about the world around us. For example, if I define a unicorn as a four-legged ungulate that has only a single horn, my definition stipulates the meaning of the term "unicorn," but it tells me nothing about whether unicorns actually exist. Similarly, deduction tells us nothing about what classes actually exist, or how they overlap with each other.


The Syllogistic Model

In deduction, the major premiss of the argument is a RULE, or general statement, while the minor premiss is a CASE, i.e. a class or instance subsumed under the rule. The conclusion is an OBSERVATION of fact, which C. S. Peirce called the "result." (I have come to prefer the term "observation," since it more accurately describes the characteristics of this statement in all three types of reasoning.) For example...

All men are mortal.        - a RULE about men.
Socrates is a man.          - a CASE - take Socrates for instance.
So, Socrates is mortal.    - an OBSERVATION - if we watch Socrates long enough, he dies.


Critique of Deductive Reasoning:

Since the validity of deductive reasoning turns upon definitions, critique of deductive reasoning would presumably take the form of critiquing the correctness of those definitions. But this hardly makes sense. We have clear general agreement on the meanings of the terms in question, so improving individual instances of deductive reasoning by improving the definitions of the sycategorematic terms used in that instance of deductive reasoning is really pretty pointless.

However, deductive reasoning, like the other forms of reasoning, is subject to a variety of fallacies. Indeed, most of the common fallacies are best understood as fallacies of deductive reasoning.


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