Starfish problems

You have heard this one:¹

One day an old man was walking along the beach. It was low tide, and the sand was littered with thousands of stranded starfish that the water had carried in and then left behind.

The man began walking very carefully so as not to step on any of the beautiful creatures. Since the animals still seemed to be alive, he considered picking some of them up and putting them back in the water, where they could resume their lives.

The man knew the starfish would die if left on the beach’s dry sand but he reasoned that he could not possibly help them all, so he chose to do nothing and continued walking.

Soon afterward, the man came upon a small child on the beach who was frantically throwing one starfish after another back into the sea. The old man stopped and asked the child, “What are you doing?”

“I’m saving the starfish,” the child replied.

“Why waste your time? … There are so many you can’t save them all so what does it matter?” argued the man.

Without hesitation, the child picked up another starfish and tossed it back into the water… “It matters to this one,” the child explained.

The version as given is as gloriously flawed as the reasoning it critiques.

What difference can we make at the margin? And relative to what?

The old man thinks the child is wasting their time because the difference they make is small relative to the size of the problem. The child is unconcerned because they make a large difference in absolute terms (many starfish!). But I am concerned because I want to know if the boy could be saving starfish more efficiently. Has he researched best-practice starfish salvation?

The question about whether our marginal difference is still worth making compared to a big problem, this seems to be tough for us to reason through.