for an aspiring teacher, would you please offer your thoughts on the statement: "those who can't do, teach."
Happy to. Unfortunately, it's sometimes true. But to my mind, if you're not doing, you have no business teaching.
I think the saying is usually applied to people who can't cut it in the real world, and so retreat to the supposedly safe world of academia, where they don't have to produce anything but can criticize their students' efforts to do so. The premise is that the competition in the world of business or work is too hot for these mediocre folks to handle, and that by teaching they are able to avoid these challenges.
I'm sure such teachers exist. But, at least at the university level, I've become convinced that they are the exception, and that whenever possible they should not be given teaching positions.
What does "doing" mean at the university level? It means performing to an audience of your peers the skills you are teaching to your students. In most cases, that's publishing and presenting at scholarly conferences. You are submitting your work for extramural evaluation, and in doing so, you are being judged in the same way you judge your students. If your work is not up to the standard to be accepted for publication by peer-refereed journals, or to be accepted at a conference prestigious enough to be selective, perhaps you are not skilled or knowledgeable enough to teach -- at least, that is the premise of these requirements for faculty promotion and tenure.
But beyond the results of these evaluations of quality, it's important to realize that the collaborative nature of the scholarly enterprise must be performed by the teacher, not just taught as an obligation to students. Students are expected to submit their work to be evaluated, the idea being that they will benefit and learn from getting feedback, from being accountable to someone other than themselves. A university teacher who is not a scholar -- who is not submitting his work to be evaluated, who is not demonstrating in performance a belief that he needs to get feedback for his own benefit and learning, who does not want to be accountable to the community of those in the same field -- is trying to make his students play by rules he does not want applied to himself.
My thinking about university teaching has increasingly moved in the direction of performance and extramural accountability. I want my students writing blogs so that they are responsible to a vast, unknown audience outside the classroom. I want them writing together, depending on each other, and having their work made public, to get them out of their isolated heads and their belief in their own heroic powers to create or fail to create alone. And I don't want to ask them to do anything I'm not doing myself -- blogging, collaborative writing, making my work public to an audience of my peers and beyond. I don't do those things because I have mastered them (with the implication that those who are learning should not be expected to do those things as an apprentice). I do them because they keep me accountable to others, enable me to move beyond what I could do alone, check my ideas and methods against those that are standard in my field, and contribute to the ongoing conversation in the areas where I presume to transmit ideas and methods to the next generation.
Some parts of the university have made performance and extramural evaluation into standard processes for undergraduates -- but they are the parts that are less traditional and most threatening to the old university ideal: business, the arts, athletics. I'm a believer in those traditional academic methods and subject matter, but I also recognize that their insularity can provide non-performing teachers with immunity from being discovered as those who "can't do." The performance of scholarship becomes a kind of game to students and faculty alike -- a hoop that has to be jumped through but isn't integral to the job, an assignment rather than an identity. The increasing reliance on non-tenure-track and adjunct faculty, those who are not expected to publish and present (perform and be evaluated outside one's own institution), threatens to make these processes seem auxiliary to teaching, separate from it, disposable or optional.
In reality, they are crucial, and students who are not taught by someone undergoing them are experiencing hypocrisy at best, and being shortchanged at worst.