XXVII. Deductive Reasoning
While inductive reasoning creates a classification based on the observation of individual objects or ideas, deductive reasoning assumes that any member has the properties of the class.
While often portrayed as opposites, deductive and inductive reasoning are complimentary. In order for there to be a class that can be used deductively, induction is used: the observation of individual objects or ideas is a prerequisite to defining a class. And so, induction takes us from particular observations to form a general theory, then deduction applies the general theory to other particular observations.
(EN: This is an assumed procedure - but it is also possible, often by secondary knowledge, to begin with the concept of a class without any individual observations. This is somewhat haphazard, but such is the nature of human thought when the heavy lifting is avoided.)
There are three essential steps in the deductive process:
- There is a schema that describes the qualities of a general class
- A specific observation is matched to some of those qualities
- The specific observation is assumed to have all of the qualities
In daily use, we are seldom aware that we are following this three-step process, but we nevertheless do so in an automatic and somewhat reckless fashion. If we are accurate in our definition of classes and our identification of items, then it generally works out for practical intents.
A syllogism is a structured series of statements - there are many technical rules and principles of logic that are beyond the scope of the present book, but the author will consider the basics.
A syllogism consists of exactly three prepositions: the major premise, the minor premise, and the conclusion.
- The conclusion represents the truth that is meant to be discovered, and must follow naturally from the logic of its premises.
- One premise must be positive ("is" rather than "is not")
- If both premises are positive, the conclusion must be positive
- If either premise is negative, the conclusion must be negative
- If one premise is particular (rather than general), the conclusion must also be particular
Again, these are not all the rules of syllogistic reasoning, but are the most major/basic ones.
All of this is to suggest that deductive reasoning takes the form of a syllogism, thus:
- CLASS has PROPERTIES
- ITEM belongs to CLASS
- Therefore ITEM has PROPERTIES
If the first and third steps in the deductive syllogism are reversed, then it is inductive logic.
Cultivation of Reasoning Facilities
There is no singular method to developing reasoning skills, but the process of practice and exercise with an awareness of method and form.
We exercise logic in daily life without realizing it, and tend to do so in a careless or at best imperfect manner - just as much as necessary to deal with the exigencies of the moment without inordinate mental effort. This leads to poorly developed logic, so it is valuable to practice logic in artificial situations outside the course of daily life to develop better reasoning skills.
Atkinson mentions a few particular fields of study that are often undertaken to practice reasoning skills: mathematics, geometry, and philosophy provide a kind of mental workout, enabling individuals to practice logical reasoning powers in a defined area. Meditating on these subjects causes the mind to develop the habit of good logical thinking, such that it may be applied to more casual and spontaneous situations.
Not only is it necessary to recognize and practice sound reasoning, but it is also necessary to recognize and avoid unsound thinking. That is, many logical "shortcuts" exist or are discovered that, while expedient, often lead us astray - the following chapter will explore some of the wrong ways of thinking.