Sunday, March 11, 2007

Better Reasoning VIII: Acceptable Premises

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Previous posts in this series have mostly focused on the formal properties of deductive arguments, especially validity. However, another important feature that has an important effect upon whether or not we should be persuaded by an argument, is the truth, or otherwise of the premises. Regular readers of this series will recall that a valid deductive argument with true premises is sound and thus should be considered persuasive.

It turns out that figuring out the truth, or otherwise of premises is a little bit more tricky than one would originally realise. Consider an argument that uses the claim that ,

"The rate of acceleration due to Gravity is 9.8 meters per second squared."

Is such a premise true? Well, the answer is really 'sort of'. A more accurate value would be 9.81 meters per second squared. Does this mean that the premise is false? Again the answer is something like 'not exactly'. The problem here is that the precision that is appropriate will rather depend upon the context.

This is not the only kind of trouble that can arise when assessing the truth of premises. This fact was tacitly recognised by Ambrose Bierce in The Devil's Dictionary, where the following cynical definition of truth was offered,

"TRUTH, n. An ingenious compound of desirability and appearance. Discovery of truth is the sole purpose of philosophy, which is the most ancient occupation of the human mind and has a fair prospect of existing with increasing activity to the end of time."

Thus, assessing premises for 'truth' is perhaps not the best way to go. A good alternative is suggested by Trudy Govier in her A Practical Study of Argument, (6th Ed.), Wadsworth (2005). Govier suggests that premises should be assessed for 'Acceptability'. I will follow Govier's suggestion here.

Govier offers a number of tests that can be used to determine whether premises are acceptable, or not. If these tests are used, then a reasonable set of strategies for assessing premises result. It is worth emphasizing that it is really important that each premise must be assessed. It is also important that one think carefully to determine whether or not a particular premise really satisfies each test. Sometimes, it can even be helpful to construct a little argument to make sure. Also, just because a premise does not appear to satisfy one of these tests, is not sufficient to ensure that the premise is not acceptable. There is a separate set of unacceptability tests, that will be discussed later.

Govier's Acceptability Tests for Premises:

- Premises defended by cogent, or sound subarguments

If a premise is defended by a strong argument, then there is every reason to judge it acceptable. This should be obvious and uncontroversial. Of course, this puts the burden of acceptability back onto the strength of the subargument and the acceptability of the premises of the subargument.

- Premises defended elsewhere

It is often the case that premises will be defended in one place, and then used in another. For example this situation frequently arises in long texts. In some way, this condition is just a variant of the previous one. This kind of justification appears in academic writing through the use of references and citation to articles in refereed academic journals.

- Premises that are known a priori to be true

The idea behind a priori judgements is that there are some judgements that can be known to be true from the concepts alone, even prior to experience. To cite a couple of classic examples, if one knows that a particular object is a triangle, then one knows a priori that this object has three sides. Similarly, if one knows that a particular individual is a bachelor, then one knows a priori that the individual is unmarried. A priori judgements are not entirely uncontroversial. However, in the current context, this can still be a useful test for acceptability.

- Premises that are commonly known to be true

There is a class of premises that are not a priori truths, but nonetheless everyone knows to be true. For instance, most people will agree that, under the appropriate viewing and climatic conditions, the sky is blue. Similarly, most people will agree that, at least in temperate latitudes (the polar region present problems here), that there is less light at night than there is during the day.

- Premises that are supported by appropriate personal testimony

This condition is useful, but can be a bit tricky. Generally speaking, accepting premises using this test, should only be done if (i) the claim is not implausible, (ii) the source of the claim appears reliable, and (iii) the claim made does not go beyond a person's reasonable experience.

- Premises that are supported by a claim to proper authority

There are experts in the world. Generally speaking, their claims can be accepted. However, it is important to consider the extent to which the authority is a reasonable and credible person in the context. For instance, a professor may make a claim about an issue. However, if the issue is outside their specialised area of training and expertise, then the claim would not be acceptable.

- Premises that are provisionally acceptable

Sometime, in the context of a real argument, it is just not possible to determine whether a particular premise is true, in the context in which the argument is being offered. In such a case, it is not unreasonable to accept the premise only provisionally. For instance, people are often doing this kind of thing when they say things like 'for the sake of argument...'.

These tests are really quite useful when used in practice. However, having a bit of experience trying using these tests can make things easier. I recommend doing a few exercises. However, once a person has become reasonably familiar with these tests, determining which arguments to accept and which to reject becomes considerably easier. Next time, the ways that premises can be rendered unacceptable will be considered.

The CP

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