With all the resources on the web these days, including free university courses, are teachers passé'? I want to argue not. Of course, that's part of my day job so I may be biased, but see if this argument holds water.
I did two degrees back-to-back in college so I had a chance to take a bunch of extra courses along the way during my six-year stint. I remember being in my fourth or fifth year when the light bulb went off and I learned how to think critically. It was in a behavioral neuroscience honors course where the two professors who taught it questioned everything and showed the students how to do this, too. It was a life-changing class--a primary thing I do as a scientist is to evaluate my own and others' research critically.
Two key elements are necessary for critical thinking. First, one must know something about the topic in order to critique what one is reading or hearing. Freshman seminars are usually not the place this happens. Second, one must be shown how to break apart an argument to find hidden assumptions, sloppy analysis or logic, and arrive at one's own conclusion. Almost know one I know learned to think critically by himself or herself. I do know of two exceptions. They are both professional magicians who must think critically or they cannot perform well.
There is probably another critical ingredient: safety. A professor or teacher can design a safe space for discussion. I do this by laying out the ground rules for arguments--taking shots at someone's idea is fine and should be done respectfully, but taking shots at a person is never allowed. When there is safety for arguments for and against, learning in a group occurs easily and rapidly.
There is some empirical support for these ideas. A recent study in the journal Psychological Science by Deanna Kuhn and Amanda Crowell randomized a group of six graders to be taught through either traditional solitary methods, or through in-class debate followed by writing essays on what they learned. They found that arguing with others is a more effective way to learn then arguing with oneself. The students, by the way, were from a low-income neighborhood and were mostly African-American and Hispanic.
The neuroscience behind the critical thinking approach is that human beings are highly social creatures, and throughout our evolutionary history as well as today, we learn naturally and easily from others. Almost no one learns alone--except for in school. There is value to learning on your own, but I encourage my graduate students to work in groups, even if I evaluate what they have learned singly.
The U.S. military makes a distinction between education and training. Pilots are trained what to do for every conceivable situation. They do not need to know more than the basic physics of why an airplane flies. They do have to drill constantly to be ready to react quickly and appropriately when something potentially dangerous occurs. Education, at its core, is learning how to think. In the truest sense, this means thinking critically, not just parroting back something that one has read in a book or skimmed off the web.
In addition to creating a safe space for discussion, I find I can facilitate critical thinking by looking at students while they speak. After years of teaching, the look in a student's face, along with her or his words, will reveal if they've come up with their own conclusions. Some students never get this, but many do, and like me I hope they are forever changed by it.