Logic in Argumentative Writing

Principles of Composition

Many of the important points of this section are covered in the section on writing Argumentative Essays: Being Logical. You might want to review that section first and then come back here for a more thorough review of the principles of logic.

This document is part of a collection of instructional materials used in the Purdue University Writing Lab. The on-line version is part of OWL (On-line Writing Lab), a project of the Purdue University Writing Lab, funded by the School of Liberal Arts at Purdue.

We use logic every day to figure out test questions, plan our budgets, and decide who to date. We borrow from the vocabulary of logic when we say, "Brilliant deduction" or even "I don't want to argue about it." In the study of logic, however, each of these terms has a specific definition, and we must be clear on these if we are to communicate.


Proposition --
T or F in an argument, but not alone. Can be a premise or conclusion. Is not equal to a sentence.
Premise --
Proposition used as evidence in an argument.
Conclusion --
Proposition used as a thesis in an argument.
Argument --
A group of propositions of which one is claimed to follow from the others.
Induction --
A process through which the premises provide some basis for the conclusion
Deduction --
A process through which the premises provide conclusive proof for the conclusion.

Argument Indicators: Premise Indicators: Conclusion Indicators:
  • should
  • must
  • ought
  • necessarily
  • since
  • because
  • for
  • as
  • inasmuch as
  • for the reason that
  • first ...
  • therefore
  • hence
  • thus
  • so
  • consequently
  • it follows that
  • one may infer
  • one may conclude
  • When dealing with persuasive writing, it will be helpful for you to outline the argument by premises and conclusions. By looking at the structure of the argument, it is easy to spot logical error.

    Example 1

    "Universities are full of knowledge. The freshmen bring a little in, and the seniors take none away, and knowledge accumulates.

    -- Harvard President A. L. Lowell

    Premise 1
    Premise 2
    Premise 3
    Freshmen bring a little (knowledge) in
    Seniors take none away
    Knowledge accumulates
    Universities are full of knowledge

    Example 2

    (Here, the conclusion of one argument is used as a premise in another. This is very common.)

    Even though there may be a deceiver of some sort, very powerful and very tricky, who bends all his efforts to keep me perpetually deceived, there can be no slightest doubt that I exist, since he deceives me; and let him deceive me as much as he will, he can never make me be nothing as long as I think I am something. Thus, after having thought well on this matter, and after examining all things with care, I must finally conclude and maintain that this proposition: I am, I exist, is necessarily true every time that I pronounce it or conceive it in my mind.

    -- Rene Descartes, *Meditations*

    Argument 1 Premise 1:

    Conclusion of Argument 1
    Argument 2 Premise 1:


    To be deceived ... I must exist

    When I think that I exist I cannot be
    deceived about that

    I am, I exist, is necessarily true ... .


    Find the Arguments and Outline them in These Statements:

    1. Ask the same for me, for friends should have all things in common.

    -- Plato, Phaedrus

    2. Matter is activity, and therefore a body is where it acts; and because every particle of matter acts all over the universe, every body is everywhere.

    -- Collingwood, The Idea of Nature

    3. The citizen who so values his "independence" that he will not enroll in a political party is really forfeiting independence, because he abandons a share in decision©making at the primary level: the choice of the candidate.

    -- Felknor, Dirty Politics

    Reaching Logical Conclusions

    This article is reprinted from pages 78-79 of Pearson-Allen: Modern Algebra , Book One. In the book it is one of several between-chapter articles that add interest and provike thought on subjects related to the topics discussed in the text.

    Consider the two statements:

    Our common sense tells us that if we accept these two statement as true, then we must accept the following third statement as true:

    We say that the third statement follows logically from the other two.

    In drawing logical conclusions it does not matter whether the statements we accept as true are reasonable or sensible. This is because we depend entirely upon the form of the statements and not upon what we are talking about. Thus, if we accept the following statements as true:

    then we must conclude that

    Try these for Fun!

    Exercises in Reasoning

    I. Four men, whom we shall call Robert, Ralph, Ronald, and Rudolph, were playing cards one evening. As a result of a quarrel during the course of the game, one of these men shot and killed another. From the facts below determine the murder and the victim.

    II. Five men are in a poker game: Brown, Perkins, Turner, Jones, and Reilly. Their brands of cigarettes are Luckies, Camels, Kools, Old Golds, and Chesterfields, but not necessarily in that order. At the beginning of the game, the number of cigarettes possessed by each player was 20, 15, 8, 6, and 3, but not necessarily in that order.

    During the game, at a certain time when no one was smoking, the following obtained:

    How many cigarettes did each man have to begin with, and of what brand?


    A functional impropriety is the use of a word as the wrong part of speech. The wrong meaning for a word can also be impropriety.

    Mark improprieties in the following phrases and correct them in the blanks at the right. If you find none, write C in the blank. Example: (occupation) hazards -- occupational

    Principles of Composition

    Guide to Grammar and Writing

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    This document is part of a collection of instructional materials used in the Purdue University Writing Lab. The on-line version is part of OWL (On-line Writing Lab), a project of the Purdue University Writing Lab, funded by the School of Liberal Arts at Purdue.