Friday, 11 May 2007

20, Argumentation Schemes for Presumptive Reasoning

Notes taken from 'Argumentation Schemes for Presumptive Reasoning' (1995), by Douglas N. Walton

1, Introduction

In accepting the (presumptive) premises, the participants are bound to tentatively accept the conclusion, for the sake of argument or discussion, unless some definite evidence comes that is sufficient to indicate rejecting it.

Such presumtively based arguments can be very useful and important in cases where action must be taken, but firm evidence is not presently available.

Practical reasoning is a kind of goal-directed, knowledge-based reasoning that is directed to choosing a prudent course of action for an agent that is aware of its present circumstances. These circumstances can change, and practical reasoning is therefore to be understood as a dynamic kind of reasoning that needs to be corrected or updated as new information comes in.

2, Presumptive Reasoning

We need to distinguish between ``concessions'' and ``substantive commitments''. A substantive commitment is a proposition that a participant in dialogue is obliged to defend, or retract, if challenged by the other party to give reasons to support it. In a word, it has a burden of proof attached to it. This is the type of commitment to a proposition that goes along with having asserted it in a dialogue. A concession is a commitment where there is no such obligation to defend, if challenged. Concessions are assumptions agreed to ``for the sake of argument''. By nature, they are temporary, and do not necessarily represent an arguer's position in a dialogue.

We note the difference between pure supposition and assertion as kinds of speech acts. Assertion always carries with it a burden of proof, becuase assertion implies substantive commitment to the proposition asserted. Supposition (or assumption) however, requires only the agreement of the respondent, and carries with it no burden of proof on either side. Presumption, as a speech act, is halfway between mere supposition and assertion. Presumption essentially means that the proponent of the proposition in question does not have a burden of proof, only a burden to disprove contrary evidence, should it arise in the future sequence of dialogue. The burden here has three important characteristics - it is a future, conditional, and negative burden of proof. It could perhaps be called a burden to rebut, in approriate circumstances.

Presumption is functionally opposed to burden to proof, meaning that presumption removes or absolves one side from the burden, and shifts the burden to the other side.

Presumption is understood as a kind of speech act that is halfway between assertion and mere assumption. An assertion normally carries with itself in argument a burden of proof: ``He who asserts must prove!'' By contrast, if a participant in argumentation puts forward a mere assumption, he or she (or anyone in the dialogue) is free to retract it at any subsequent point in the dialogue without having having to give evidence or reasons that would refute it. Assumptions are freely undertaken and can be freely rejected in a dialogue.

In order to be useful, presumptions must have a certain amount of ``sticking power'', but by their nature, they are tentative and subject to later retraction.

For example, in a potentially hazardous situation, it may be prudentially wise to tilt the burden of proof in the direction of safety. The maxim is to ``err on the side of safety'', where doubt creates the potential for danger.

A simple case is the accepted procedure for handling weapons on a firing range. The principle is always to assume a weapon is loaded, unless you are sure that it is not loaded. The test of whether you are sure of this is that you have, just before, inspected the chamber and perceived clearly that it is empty.

The same kind of example shows also, however, how tied to the specifics of a context or situation this kind of reasoning is. Suppose you are a soldier in wartime getting ready to defend your position against an imminent enemy assualt. Here, reasoning again on practical grounds of safety or self-preservation, you act on a presumption that your weapon may be empty, by checking to see that it is not empty.

Customs, fashions, and popularly accepted ways of doing things, are another important source of presumptions. With many choices on how to do things in life, in the absence of knowledge that one way of doing something is any better or more harmful than another, people often tend to act on the presumption that the way to do something is the popularly accepted way of doing it.

3, The Argumentation Schemes

Walton describes and analyses 25 different argumentation schemes. For each argumentation scheme, a matching set of critical questions is given. This pairing brings out the essentially presumptive nature of the kind of reasoning involved in the use of argumentation schemes, and at the same time reveals the pragmatic and dialectical nature of how this reasoning works. The function of each argumentation scheme is to shift a weight of presumption from one side of a dialogue to the other. The opposing arguer in the dialogue can shift this weight of presumption back to the other side again by asking any of the appropriate critical questions matching that argumentation scheme. To once again get the presumption on his or her side, the original arguer (who used the argumentation scheme in the first place) must give a satisfactory answer to that critical question.

Some of the argumentation schemes are basic or fundamental, whereas others are composites made up from these basic schemes.

4, Argument from Ignorance

The arguments associated with these argumentation schemes are typically used in a balance of considerations type of case, where knowledge or hard information is lacking, of a kind that would enable the problem to be resolved or the dispute to be settled on that basis. In other words, these presumption-based arguments are generally arguments from ignorance. The logic of these arguments could be expressed by the phrase, ``I don't know that this proposition is false, so until evidence comes in to refute it, I am entitled to provisionally assume that it is true.'' All of the argumentation schemes previously studied tend to take this general form.

In some cases, the argument from ignorance is a correct (nonfallacious) argument because we can rightly assume that our knowledge base is complete. If some proposition is not known to be in it, we can infer that this proposition must be false.

5, Ignoring Qualifications

6, Argument from Consequences

The argument from consequences may be broadly characterised as the argument for accepting the truth (or falsehood) of a proposition by citing the consequences of accepting (rejecting) that proposition

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