Let’s say I’m addicted to prescription pain-killers. You are my concerned friend. “Charles,” you say, “you’ve really got to get off this medication. It’s ruining your health, and someday you’re likely to OD.”

“But I can’t stop taking it. I’m in pain all the time. If I don’t take it I can’t function at all. I have terrible back pain, and my doctor says there is nothing I can do about it.”

If you accept the premises of my response, you’ll have little to say. If we both accept that there is no other way to reduce the pain, and that the cause of the pain is incurable, then I’m right, I have to keep taking the painkiller.

Now let’s talk about glyphosate, the much-maligned herbicide that Monsanto markets as Roundup. Critics make compelling points about its effects on human and ecological health. Defenders rebut those points, at least to the satisfaction of regulators. The debate has raged now for decades. One point that Roundup’s defenders make is this: “Look, Roundup is the most effective broad-spectrum herbicide we have. If we stop using it, crop yields would fall. We would have to use other, less effective herbicides that might be even more toxic to human beings and the environment. Roundup is the safest, most economical option available.”