What is relevant when we try to help people learn mathematics.
In spite of the seemingly infinite amount written by Psychologists and Educologists on learning---not to say on teaching, the amount of any value to those of us who are "in the trenches" turns out to be remarkably limited. Here are the only four people I think have made valuable contributions towards what we can do---and what we should definitely not do---while trying to help people learn something other than "mathematical facts and skills".