Tom: Tell me, do you think that a single grain of wheat is a heap?
Mathhead: Certainly not.
Tom: And do you agree that adding a single grain could never turn a non-heap into a heap?
Mathhead: Kind of. I think there is a middle ground between 'heap' and an emphatic grade of 'non-heap', which we shall call a 'pittance' of grains, at the other extreme. And I do declare that adding a single grain will never turn a pittance into a heap.
Tom: Ok. Now, is a single grain anything other than a pittance?
Mathhead: I think that no (zero, the absence of) grains of wheat is always a pittance.
Tom: Well, good. But I didn't mention zero. What about the single grain? Is that not equally a pittance?
Mathhead: [Please confirm, yes, a single grain is a pittance!! Or I'm very confused.]
Tom: And are you comfortable with the addition of a single grain turning a pittance into a non-pittance? If so, are you willing to locate the lower threshold M (going from pittance to non-pittance)?
Mathhead: I'm comfortable with the idea of a threshold in principle, but it's naive and, perhaps, missing the point to think that we could or should try to locate it. It's too volatile for that.
Tom: On the other hand, is it so volatile that we can't assume it would never reach down to a single grain?
Mathhead: [hoping for an absolutist answer here, but I shan't presume it this time...]
Tom: [... and you can guess where I want to go from there...]