FORMAL AND INFORMAL LOGIC

INTRODUCTION

Reasoning is one of the chief characteristic of a normal human person, and logic has it at its center of discipline the science of good reasoning. In this paper we shall be looking into what logic is all about with special reference to the two main branches of logic.

Definition of logic

Logic is the study of the methods and principles by which we differentiate good reasoning from bad reasoning, correct reasoning from incorrect reasoning.[1] It is equally the study of the principles and techniques of distinguishing good arguments from bad arguments.[2] We can say that Logic is the branch of philosophy which reflects upon the nature of thinking itself.[3] It is divided into two, formal and informal logic.

Formal Logic

 

Formal logic comprises those areas of logical theory that study the forms of reasoning independent of the content of reasoning.[4] It is thus by its nature quite abstract, for it abstracts the form from the content.[5] It is sometimes referred to as symbolic logic because it uses special symbols and formulas, similar to those used in mathematics to represent the forms of reasoning. Since logical forms are abstract, they are well suited to symbolic expression.[6]

 

Informal Logic

Fisher and Scriven defined informal logic as "the discipline which studies the practice of critical thinking and provides its intellectual spine". By "critical thinking" they understand "skilled and active interpretation and evaluation of observations and communications, information and argumentation.[7]"

Ralph H. Johnson and J. Anthony Blair define informal logic as "a branch of logic whose task is to develop non-formal standards, criteria, procedures for the analysis, interpretation, evaluation, criticism and construction of argumentation.[8]

Informal logic according to Frans H. van Eemeren, covers a "collection of normative approaches to the study of reasoning in ordinary language that remain closer to the practice of argumentation than formal logic.[9]"

The three most important branches of informal logic include:

1. The study of definitions
2. The study of the informal fallacies
3. The study of inductive reasoning.

Comparisms of formal and informal logic

1.      Formal and informal logic all deal with arguments; the formal logic is concerned with the form, while the informal logic is concerned with the reasoning

2.      They all deal with fallacies; formal and informal fallacies

3.      They are all geared towards making a better argument

4.      They all study arguments, formal logic studies the form while informal logic studies the content

5.      They are all necessary in identifying fallacies, formal and informal fallacies.

 

Contrast of formal and informal logic

 

 

1.      Informal logic analyzes the grounds or reasons for conclusions. It looks at how well reasons support, justify, establish or demonstrate in some way, the conclusion. This typically involves questions of degree, probability, plausibility and persuasiveness while, Formal logic is concerned with how the components of an argument relate to each other. It focuses on the formal rules for the arrangement of statements that may guarantee the validity of an argument. It is thus heavily concerned with ‘form’.[10] Example,

All Igbos are Nigerians

All Nigerians are Africans

Therefore, all Igbos are Africans (valid)

All Igbos are Africans

All Nigerians are Africans

Therefore, all Igbos are Nigerians (invalid)

A formal argument can have a valid logical form, but consist of completely false propositions.[11] For example:

All fish have lungs

Whales are fish

Therefore whales have lungs

2.      Informal logic involves the analysis, evaluation and interpretation of arguments made in natural language, (in real life speech situations). It involves the logic of argument, as opposed to the logic of deductive inference[12], while Formal logic deals with propositions that have absolute truth values, inferences that have precise standards of validity, and typically involves 'deductive reasoning'.[13]

3.      Informal logic deals with inductive reasoning, (reasoning from particulars to the general, and in which the premises provide some degree of support for the conclusion).[14] While Formal logic deals with deductive reasoning, that is, the premises provide full support for the conclusion.

4.      Informal logic has no strict criteria by which to judge arguments. Formal logic focuses on the formal rules for the arrangement of statements that may guarantee the validity of an argument.[15]

5.      Informal logic deals with the following; the quality of the support used to justify a claim; the nature of the evidence/examples used; the strength of the premises; the nature of the assumptions that underlie the argument; the nature of the implications or consequences the argument leads to; the internal consistency of the claims made; the way authorities are appealed to; how well the argument anticipates and deals with alternative positions; how well the argument handles counterexamples and counterarguments, and the nature of the audience etc.[16] while formal logic is strictly deductive, that is, the premises provide full and absolute support for the conclusion.

6.      The form of an argument rarely tells us much about how persuasive or reasonable an argument is[17], while informal argument does.

REFERENCE

     

I.M. copi, introduction to logic,6^th edition,(New York: Macmillan,1990) 

R.N. popkin & A.Stroll. philosophy made simple,(London: Heinemann,1982)

Paul herrick, http://www.manyworldsoflogic.com/informallogic.html 2011

http://eserver.org/courses/spring2000/76101c/logic.html

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[1] I.M. copi, introduction to logic,6^th edition,(New York: Macmillan,1990).page 6

[2] ibid

[3] R.N. popkin & A.Stroll. philosophy made simple,(London: Heinemann,1982) page 3

[4] Paul herrick, http://www.manyworldsoflogic.com/informallogic.html

[5] ibid

[6] ibid

[7] wiki

[8] ibid

[9] ibid

[10] http://eserver.org/courses/spring2000/76101c/logic.html

[11] ibid

[12] ibid

[13] ibid

[14] ibid

[15] ibid

[16] ibid

[17] ibid