An ardently humane correspondent

 It makes the greatest sense to me, to look to WJ's letters for insight into both his personal temperament and his core philosophical commitments. Those were inseparable for him, comfortably so. Or more comfortably than for most philosophers, as he indicates in The Present Dilemma:

The history of philosophy is to a great extent that of a certain clash of human temperaments. Undignified as such a treatment may seem to some of my colleagues, I shall have to take account of this clash and explain a good many of the divergencies of philosophers by it. Of whatever temperament a professional philosopher is, he tries when philosophizing to sink the fact of his temperament. Temperament is no conventionally recognized reason, so he urges impersonal reasons only for his conclusions. Yet his temperament really gives him a stronger bias than any of his more strictly objective premises.

 We don't really write letters anymore, not like people did in James's day. If we did we'd still come nowhere near, most of us, the mark of playful delightful combined with wit and intelligence his most casual correspondence typically met. As his son said in the preface to the first abridged volume of his letters, "he could not write a page that was not free, animated, and characteristic." 

That went for missives to friends, family, and relative strangers alike. But you get the impression that no one remained a stranger to him long, or felt estranged. Even those who knew him only through his letters thought they knew him intimately, "it was plain to everyone who knew him or read him that his genius was ardently adventurous and humane."

He was already a literary charmer at age nineteen, writing to his family from school "a resume of [my] future history for the next few years... Thus: one year study chemistry, then spend one term at home, then one year with Wyman, then a medical education, then five or six years with [naturalist explorer] Agassiz, then probably death, death, death with inflation and plethora of knowledge. This you had better seriously consider. This is a glorious day and I think I must close and take a walk. So farewell, farewell until a quarter to nine Sunday evening soon! Your bold, your beautiful, Your Blossom!!"

A bit florid, sure, but he's a teenager for heaven's sake. He doesn't yet know he's a philosopher and a blossoming peripatetic.

I think I too must close now and take a quick walk with the dogs before heading to school. Time is ever shorter and I can't quite joke around anymore about death, death, death.