From time to time I will explore in depth a particular logical fallacy. Logic is a common theme in this NeuroLogica blog, and a finely tuned understanding of logic both valid and fallacious is essential to critical thinking. But fallacious logic is not always simple to understand and apply to the real world, so it is helpful discuss examples and applications. I claim no expertise in logic, I just like to use it as often as possible, and it’s kind of hard to avoid being an active skeptic. Today’s fallacy is the argument from personal incredulity.

The essence of this fallacy is the argument that a particular claim is not true because it is difficult to imagine, understand, or believe. What I consider to be a subset of this fallacy is the argument from ridicule – that something is wrong because it is ridiculous.

This does not mean that every statement that a claim is ridiculous, or that it is unbelievable, is committing this fallacy. It is only the argument that a claim is wrong because it is unbelievable that is invalid. If, rather, one argues that a claim is ridiculous because it lacks evidence, plausibility, or probability – that is not a fallacy. One must pay close attention to the structure of the argument – what the logical connection being made is – before deciding if it is valid or not.

Also, an argument that a claim is highly improbable (as long as the probability assessment is backed up with its own arguments) can be valid, and is not an argument from incredulity.

Here are some examples.

Creationists often argue that evolution is impossible because life is simply too complex to have evolved by chance alone. They sometimes will give a specific example, such as the eye. But what does “too complex” mean? I have not seen any specific definition, or any way to estimate how much complexity evolution could produce. The statement is nothing more than an argument from personal incredulity.

But creationists also make many similar claims that may seem superficially like the argument from personal incredulity but they are not. Here is a typical example from a creationist website: “Actually, given the enormous complexity of a single cell, I believe that probability theory tells us that we cannot assume that molecules in the sea of Earth have spontaneously have formed a single cell by chance.” This argument attempts to justify an argument of non-plausibility by making an estimate of probability. In fact, sometimes an actual calculation is used. Fred Hoyle calculated of the probability of the spontaneous origin of 2000 proteins of 200 amino acids each at 10-40000. This figure is often cited by creationists as the odd against a single cell of this complexity arising by chance. Some go the extra step to argue that there was insufficient time, even giving the billions of years of the Earth, for such a low probability event to have occurred.

This is not an incredulity argument, it is a probability argument. It also happens to be a straw man – biologists do not believe that a cell with this amount of complexity arose spontaneously. Such complexity likely took hundreds of millions and perhaps billions of year to evolve. Since evolution is a process by which specific complexity may accumulate non-randomly, the probability argument is worthless. Current thinking is that a crude self-replicating molecule (the precursor of RNA and DNA), perhaps with some proteins and raw materials, became trapped inside a spontaneously forming bubble of bilipid membrane. Lipids (fats) that have a hydrophobic (water fearing) end and a hydrophilic (water loving) end form spontaneously in water into bilipid layers – two layers with the hydrophobic ends together and the hydrophilic ends pointing outward. Such layers form spontaneously into bubbles. Such a bilipid bubble is the basic membrane of all cells.

Such a primitive lipid bubble likely would have provided an advantage for the self-replicating molecules inside, because they could sequester their raw materials for replication close at hand, as well as the proteins whose formation they direct from amino acids. Those replicating molecules that caused proteins to come into existence that offered them a survival or replicating advantage would tend to survive.

In other words – once you have a toe-hold on evolution, calculations of forming spontaneously by chance do not apply. Even primitive cells evolved over a very long period of time.

On the other hand, skeptics often make legitimate probability arguments, and these can falsely be dismissed as arguments from incredulity. We certainly also ridicule absurd propositions – but again this is not fallacious as long as independent justification is offered. Recently I have been discussing on the SGU boards the argument that one of the main problems with 9/11 conspiracy theories is that they are intrinsically improbable.

Granted there are many assumptions in any estimation of the probability of pulling off an elaborate conspiracy without getting caught. But logical inferences can be made to create a valid argument that the scope and complexity of 9/11 is such that the odds of carrying it out without getting caught are vanishingly small. As long as the inferences are based upon the best available evidence and are reasonable, the conclusion can be valid and reliable. The key is that the judgment is not based upon a dismissive characterization of absurdity, or on the inability to imagine how it could have been pulled off, but rather a reasonable assessment of what would have been required at minimum for such an operation to be successful.

I hope you found this helpful. Feel free to disagree and argue with me about any of the particulars. I find the best way to exercise our logic circuits is to engage in vigorous debate. Just remember to analyze your own logic as carefully as you do other’s, and keep focused on the logic and evidence rather than needless speculation about motivation or personal attributes.