|He may be bad at reasoning, but Evil Spock has an amazing goatee.|
As it has been pointed out by Eliezer Yudkowsky, Julia Galef (who is the first to use the following two examples), and Luke Muehlhauser, there is nothing rational about Spock's decision making process. For whatever reason, Gene Roddenberry wrote our Vulcan hero as someone who does not take other sentient being's emotion into account when he is deciding what to do. To see what I am talking about, examine this dialog between Spock and McCoy on the the episode Galileo Seven:
McCoy: "Well, Mr. Spock, [the aliens] didn’t stay frightened very long, did they?"The type of rationality that Spock is betraying here is known as epistemic rationality. This kind, also known as type 1 rationality, is our commitment to having as accurate a view of the world as we possibly can. If Spock were abiding by a commitment to know the world as it really is, then he would have been obligated to take into account all of the evidence about behavior even if it went against his preconceived notions of logic. By not doing so, he is being the very definition of delusional (having a sustained disregard of reality).
Spock: "A most illogical reaction. When we demonstrated our superior weapons, they should have fled."
McCoy: "You mean they should have respected us?"
Spock: "Of course!"
McCoy: "Mr. Spock, respect is a rational process. Did it ever occur to you that they might react emotionally, with anger?"
Spock: "Doctor, I’m not responsible for their unpredictability."
McCoy: "They were perfectly predictable, to anyone with feeling! You might as well admit it, Mr. Spock: your precious logic brought them down on us!"
The other type of rationality that Spock has a hard time grasping is instrumental rationality (or type 2 rationality). Unlike epistemic rationality, instrumental rationality is about applying your knowledge to a task and finding the most reliable and efficient way of completing it. For example, on the episode Charlie X, Spock loses in chess to Kirk and declares "Your illogical approach to chess does have its advantages on occasion, Captain." But Kirk's playing style is a textbook case of instrumental rationality. This is because, if Kirk's goal is to win at chess, his methods allow him to consistently accomplish what he desires.
Spock also goes against instrumental rationality when he downplays the importance of passion. While it is certainly true that passion can cloud one's judgement and lead to an overestimation of one's own prowess, it also motivates us to dream up and embrace many of our goals. For example, being passionate about the stars motivated Vincent van Gogh to paint and Galileo Galilei to turn his looking glass to the sky. This is what David Hume meant when he said "Reason is, and ought only to be the slave of the passions."
In closing, Spock has the wrong idea about rationality. Clear thinking is about abiding by two types of rationality. The first is having the most accurate map of the world possible, while the second is about reliably and effectively achieving your goals. This is very different than reasoning devoid of emotions or saying esoteric dictums about decision making. If you want to learn the actual methods of critical thinking, stay tuned to this series. In the meantime, live long and prosper.