Q:  What's the most important leg of a three-legged stool?

A:  The one that isn't there.

- traditional joke-riddle

A specific neurological lesion can sometimes damage or impair specific neurological functions without touching others.  In the condition famously known as "Ondine's Curse", for example, automatic control of breathing is destroyed while conscious control remains, so that without modern medical intervention nerve-damaged patients can survive only as long as they can remain awake.  Such conditions are nevertheless unusual exceptions to the more general principle that complex, recently-developed, and 'meta'-functions (those that monitor and control others) are first to be impaired and lost when the nervous system is stressed, damaged, or altered.

Demonstrations of this principle can be found by examining such phenomena as reversion under stress, oxygen deprivation, sleep deprivation, and various sorts of poisoning - most especially drugs with a gradual effect on nervous function.  The primary reason it is considered necessary to have designated drivers who refrain from consumption of alcohol is that drinkers frequently underestimate the degree to which they're affected by alcohol.  Long before slowed reflexes and grossly impaired judgment become evident, the cognitive functions responsible for self-evaluation are dulled, and self-control diminished.  A drinker who believes that they're capable of driving safely may or may not be correct, even if their judgments would normally be trustworthy.  Similar effects are found with other types of intoxication - people who say that they drive better after smoking marijuana have been shown to in fact drive more poorly.  The more demanding and complicated the mental task is, the more likely it will be disrupted by any interfering factor, leading to poor performance. 

The natural human's an animal without logic. Your projections of logic onto all affairs is unnatural, but suffered to continue for its usefulness. You're the embodiment of logic--a Mentat. Yet, your problem solutions are concepts that, in a very real sense, are projected outside yourself, there to be studied and rolled around, examined from all sides."
"You think now to teach me my trade?" he asked, and he did not try to hide the disdain in his voice.
"The finest Mentats have a healthy respect for the error factor in their computations," she said.

- exchange between the Lady Jessica and Thufir Hawat;  Frank Herbert,  Dune

Once a certain level of intelligence has been reached, any cognitive process can be emulated by any mind - it's merely a question of available storage space and speed.  The amount of processing capacity rapidly becomes immaterial.  What's important is not how powerful a mind is, but how well it detects, compensates for, and corrects its own errors.

It would be convenient if this capacity, which is difficult to gauge, were clearly associated with general intelligence.  Unfortunately, that doesn't seem to be the case.  We know that the self-regulatory functions of cognition can be completely destroyed without affecting such things as IQ scores.  Thus, high IQ does not serve as a reliable guide to the presence of higher cognitive functions.   Furthermore, my experience with smart people strongly suggests that they are less likely to develop that capacity.  Being cleverer than the people around them, they are more likely to be able to craft invalid yet convincing arguments that others can't counter or respond to.  They have no need to develop stringent self-evaluation to accomplish social goals, and it's very easy to convince themselves that they've chosen the correct course of action.  What's worse, they're more likely to be able to craft clever arguments which convince themselves - and then, secure in the knowledge of their cleverness, they become less likely to check and re-check their reasoning.  Average people have more experience of being shown to be wrong, and often have developed a greater willingness to lack confidence in their conclusions.  This is both a strength and a weakness, but the strength cannot be acquired otherwise while the vulnerability can be compensated for.

"The first principle is that you must not fool yourself - and you are the easiest person to fool." - Richard P. Feynman

Rationality, like reading or arithmetic, is a skill alien to the human mind.  Useful, certainly, but not natural.  Development of the capacity for rationality requires strict adherence to a set of formal principles, and such adherence requires advanced self-evaluation to be maintained.  Otherwise practitioners will quickly convince themselves that short-circuited thinking really is valid.  If it's a terrible thing to believe your own propaganda, it's even worse to never realize you're issuing propaganda in the first place.

The principles of rationality aren't difficult.  What's hard is to implement them consistently and completely; our older, better-developed tendencies to associate our way through a problem and accept or reject statements on the palatability of their consequences, tend to override our better judgment.

How, then, can we develop the ability to put rational thought into practice?