Haruki Murakami writes the preface. What Mr. Murakami says is quite interesting. I quote.
-----------------
The protagonist of Sanshiro, however, is different. He, too, is unable to find his proper place amid dislocated circumstances, but he never fully confronts those circumstances as a problem within himself. Instead, he accepts them in a relatively natural way, with a young man's particular kind of nonchalant resignation, as something entirely external to himself. "Oh, well, that's how it goes," he seems to say. Stomach pain has not yet entered his world. I think that Sanshiro is a personal favorite of mime because it depicts this natural functioning of the young protagonist's psyche in an utterly mellifluous style. Sanshiro watches life sweeping him along the same way he looks at clouds sailing through the sky. The free movement of his gaze draws us in almost before we know it, and we forget to view him critically.
Haruki Murakami, Preface to Sanshiro, translated by Jay Rubin.
-----------------
Here, Mr. Murakami's usage of the "stomachache" metaphor is interesting. It is well known that Soseki Natsume suffered from stomach conditions for most of his career, especially in the late years. It may well be that such physical circumstances influenced what Soseki wrote as a novelist in a profound way, affecting the world view and the manner in which the protagonist of the story moved around in the fictitious world.
In this context, Haruki Murakami himself might be regarded as "Soseki without stomachache".
Sanshiro. Translated by Jay Rubin.