The depth of everyone's knowledge varies from subject to subject. I have a deep understanding of many areas of securities law, but a very shallow understanding of physics. (I’m not even in the wading pool.) But, even in subjects I teach—business associations, securities law, accounting for lawyers—the depth of my knowledge varies from topic to topic.

When I’m teaching the Securities Act registration exemptions, my knowledge base is very deep. I research and write primarily in that area. I know the law. I know the lore. I know the policy.
In other areas, my knowledge is much shallower. In some cases, I know just enough to teach the class. My business associations class sometimes touches on entity taxation issues, but I’m far from an expert on entity taxation. (My tax colleagues would say “far, far, far.”)

One’s knowledge deepens over time, of course. That’s one of the great joys of becoming an expert, whether you’re a law professor or a practitioner. I know more now about every topic I teach (including entity taxation) than I knew when I began teaching 27 years ago.

Several years ago, I decided to teach a course on investment companies and investment advisers. I started from scratch. I had no such class in law school and I didn’t practice in that area, so I had to learn the details myself before teaching the class. Now, having taught the class many times and having written two articles that deal with issues in the area, my knowledge base is much deeper.

All law professors have shallow and deep areas of knowledge. Over time, all of us should try to deepen our knowledge in the shallower areas. This improves our teaching and, less obviously, improves our scholarship. I tell my students that a broad education benefits the specialist, and my own experience confirms that. I have often drawn on what I learned in one of my shallower areas while writing an article in a deep area.

Professors also need to be careful that our teaching isn’t negatively affected by our shallow and deep areas.

• Be sure your course coverage (and your exam coverage) is based on the importance and relevance of the topics and the needs of the students, not on your knowledge base. There’s a natural psychological tendency to focus on what we know best, which is usually also what we’re most interested in. Don’t minimize a topic just because your knowledge of the topic is shallow. Don’t stress a topic just because your knowledge is deep. I would like to spend my entire securities regulation course talking about Securities Act exemptions, but I don’t.

• Be careful to maintain the same classroom atmosphere in shallow and deep areas. When I’m teaching in a deep knowledge area, I’m often just scratching the surface of what I know. I sometimes have to fight to stay excited about the material and avoid going on autopilot. When I’m teaching in a shallow area, the discussion is fresher and more exciting to me. I’m more likely to learn from my students and I can empathize with their struggles to master the material. The key is to keep an even keel—to keep the discussion equally fresh and exciting, no matter how deep or shallow your knowledge.

• Don’t overwhelm the students with your deep knowledge. They need to spend some time in the shallow end before you can take them into the depths. It’s taken you years to develop your deep knowledge; you can’t replicate that for your students in an hour or two.

• Admit when your knowledge is shallow. “I don’t know” is a perfectly appropriate response even when your knowledge is deep, even more so when your knowledge is shallow. And “I don’t know” is much better for you and your students than trying to fake it. Use these opportunities to deepen your knowledge and get back to the students with your answer. I can’t count how many times in my career I have faced situations like that.

I apologize for disillusioning any readers who, based on this blog, believed I was omniscient and had deep knowledge of everything.