My original title for this lecture was “Aspects of Accessibility.” Then I discovered that people assumed it concerned accessibility in connection with aircraft maintenance. That assumption implied that an engineer is expected to stick to engineering. Having written what is to follow, I wonder whether that implication might well be right, and that all I have to say could be entitled:

On the other hand G. K. Chesterton clearly demonstrated that when the obvious is stressed, it often turns out to have unexpected aspects.