Hermann Hesse's Steppenwolf discusses this quote which it attributes to Novalis.

In the book's introduction, Harry Haller mentions the quote to his landlord's son one day in passing and the two discuss.

"Most men will not swim before they are able to"

A good point. Man was not built to swim, he was "built for the solid earth". Logically then, man will never swim, since one learns by doing.

Haller pursues the idea further.

Man was built for the tangible, so just like he won't swim he will not think, he was built for (and from) the solid earth, he was not made for the realm of thought.

Then who are all these people swimming?

Is it man was not supposed to swim, or man is the most adaptable creature on the earth? For example, before man really evolved to what we are today, running out into the water might have seemed like a good idea to get away from some dangers on land, but maybe only a few took the chances to actually learn how to swim. Thus, passing it off to any other person helping them escape. Some who don't know how to swim might die leaving the swimmers around. Yes, at the early ages of childhood children are afraid of water, but swimming is something man can adapt to with practice.