Cos X says so

Many arguments fail in the very first premises. They fail because they start out with an essential assumption that is not supported either by empirical evidence, human experience, or logic.

One of the earliest examples of such a bare assertion fallacy is found in Plato's First Cause.

Such fallacious arguments often rely upon the fact that by the time that many readers or listeners reach the conclusion, they have forgotten that the first premises were unreasonable. Creationists and religionists often rely upon waffle to disguise their fallacies of logic.

The bare assertion fallacy could be called the "cos-X-says-so fallacy".
Premise 1: X claims statement A.
Premise 2: X claims that X is not lying.
Conclusion: Therefore, A is true.

The commonest examples of this fallacy refer all assertions back, you guessed it, to scriptural dogma.

Premise 1: The Bible says A
Premise 2 (explicit or implicit): The Bible is the Word of God, and God would not lie, or God knows everything (common bare assertions)
Conclusion: Therefore, A is true

When religionists wish to reject the literal word of the Bible
Premise 1: The Bible does say B
Premise 2: But, you simply do not know how to interpret the Bible correctly
Conclusion: The Bible is the literal Word of God and you must accept that A is true

Premise 1 would be acceptable If X has indeed made statement A, but
X has backed up the claim with empirical evidence or with a valid argument,
X has made an acceptable claim with which all reasonable people can agree (the sun rises in the east)

A common variant is the I-say-so fallacy.

I claim that D, or if anti-C then D
. . .
. . .
Therefore, anti-D is true

Here’s an example:

Veritas48's / Alvin Plantinga's argument, linked from here:

1. If naturalism and evolution are true then the probability of our cognitive abilities to
be reliable is low.
2. If the probability of our cognitive abilities to be reliable is low then the probability
of any belief we arrived at using our cognitive abilities is low.
3. People who believe that N&E are true have arrived at this belief using their cognitive
4. Therefore given N&E the probability of the belief in N&E being true is low.
5. Therefore the idea that N&E are both true can be rejected.

The problems:
With the argument: If our cognitive abilies are indeed low and unreliable, then so must the cognitive abilities of the arguer be low and unreliable. Thus, the argument should be rejected on the grounds of unreliability. (This appears to be the case, but not for the stated reasons).

With Premise 1, in particular: Our cognitive abilities are the cognitive abilities that we actually possess, regardless of the mechanism by which we aquired these abiliities. This means that those who accept naturalism and evolution as explanations should be, on average, equally cognitively capable as those who reject naturalism and evolution. (In fact, abundant evidence indicates an inverse correlation between intelligence and rejection of naturalism and evolution. However, that is not the point here.)

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