What's the point of grades?

Excerpt from "A Thought Experiment: Why Grade? Why Test?"

Let’s try a thought experiment.   Let’s assume we live in a culture where all forms of educational achievement tests have been banned and no one is allowed to assign a letter or numerical grade for anything.   How would we evaluate what students are learning?  How would we decide which teachers were doing their job effectively or how they could be more effective?  Would there be objective (i.e. impartial, unbiased) ways of determining who was the smartest student and who needed help?   And why would we want or need to know that?  Without testing, would being the best be a useful question?   Or, as a mathematician would ask, would that question be an interesting one (one that could yield an answer that wasn’t simply a circular restating of the question)?  How would the content and methods of education change if assessment by means of testing and grading was banned?

One uses the method of a thought experiment, or gedanken experiment, when one wants to challenge a paradigm that is so foundational and so entwined with current assumptions that you need to move outside the realm of the experimental, experiential, and the plausible even to see it.   Much of modern physics, going back at least to Galileo, is founded on thought experiments, often subsequently demonstrated empirically (as in dropping objects off the Leaning Tower of Pisa to demonstrate new theories of gravitation).   A thought experiment is the only way to unravel the current state of thinking from all of the baggage that, over time, becomes not only associated with that mode of thinking but becomes foundational to it. Reform rarely leads to a paradigm shift since it builds upon these interwoven histories.  You use a thought experiment when you want to change not just a way of measuring the world but how we see it.