A fellow I know was interested in category theory. I said I might have a book for that. I found Conceptual Mathematics by F. William Lawvere and Causality by Judea Pearl (because it also looked interesting) in my personal library and handed them to my friend. He lives seven hundred miles away and might not get them back to me for a while if ever. He and I know this.
The principle is clear. The books were gathering dust on my shelves and books are meant to be read. But I felt a painful twinge. The books contain ideas that I will never master or marvel at or use. They were on my shelves. I had read them – at least parts and scanned the rest but their ideas and their author’s organizing vision will never be part of my life as great, as interesting, as wonder-full as they are. By lending the books I was admitting that I could not know all things mathematical (of course), that I was limited in time and energy, that I was getting old. The lending act was evidence that I was giving up and it hurt.