Teaching creates two contrary mentalities — ally and adversary. Good teaching requires embracing that dual nature.

Einstein wrote about the wave–particle duality:

It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.

In teaching there is likewise a duality for the teacher. Sometimes students experience the teacher as a gatekeeper; other times, as a friend. Or as Peter Elbow writes in Embracing Contraries: Explorations in Learning and Teaching (Oxford, 1986) [pp. 142–3, condensed]:

Good teaching is hard because it means trying to be an ally and adversary of students. The two conflicting mentalities needed for good teaching stem from the two conflicting obligations inherent in the job: we have an obligation to students but we also have an obligation to knowledge and society. Our loyalty to students asks us to be their allies and hosts as we instruct and share: to invite all students to enter in and join us as members of a learning community. We also have a responsibility to society—that is, to our discipline, our college or university, and to other learning communities of which we are members—to see that the students we certify really understand or can do what we teach, to see that the grades and credits and degrees we give really have the meaning or currency they are supposed to have.

How can teachers endure these conflicts? Elbow suggests “making peace between opposites by alternating between them so that you are never trying to do contrary things at any one moment” [p. 152]. That means teasing apart the roles of instructor and evaluator:

• As an evaluator, set clear expectations at the beginning of the semester, and in each assignment and exam. Carefully explain how assessments and rubrics will work. Be wholehearted and enthusiastic about you standards.
• Then as an instructor, commit to helping students attain those standards. Be a kind of lawyer for the defense, helping bring out the best in students in their battles against the other you, the prosecuting attorney.

The phase distinction here enables the teacher to simultaneously have high standards and great supportiveness. You can feel better about being rigorous in assessment if you know you are also going to be rigorously supportive while preparing students for that assessment. There is no need to hold back or to feel too soft. [p. 155, condensed]

The best way to be supportive is to be a kind of coach. The coach identifies, “you’re strong here, you’re weak there. We’ve got to find ways to work on the weaknesses so you can succeed.” That model helps reward students for volunteering weaknesses. The teacher can ask, “what don’t you understand? What skills are hard for you? I need to decide how to spend our time here and I want it to be the most useful for your learning.” [p. 156]

The coach can be both instructor and evaluator, gatekeeper and friend, a good teacher: dual in nature, but not conflicted about the contrary nature of the job.