Chuang Tzu, a brilliant fourth century BC thinker and writer, uses the image of the "useless tree" several times in the opening chapters (referred to by those in the know as the Inner Chapters) of his book, which is named for him: Chuang Tzu. Here's Lin Yutang's translation, which is not my favorite but is easily accessible on the web:
A certain carpenter Shih was travelling to the Ch'i State. On reaching Shady Circle, he saw a sacred li tree in the temple to the God of Earth. It was so large that its shade could cover a herd of several thousand cattle. It was a hundred spans in girth, towering up eighty feet over the hilltop, before it branched out. A dozen boats could be cut out of it. Crowds stood gazing at it, but the carpenter took no notice, and went on his way without even casting a look behind. His apprentice however took a good look at it, and when he caught up with his master, said, "Ever since I have handled an adze in your service, I have never seen such a splendid piece of timber. How was it that you, Master, did not care to stop and look at it?"
"Forget about it. It's not worth talking about," replied his master. "It's good for nothing. Made into a boat, it would sink; into a coffin, it would rot; into furniture, it would break easily; into a door, it would sweat; into a pillar, it would be worm-eaten. It is wood of no quality, and of no use. That is why it has attained its present age."
When the carpenter reached home, he dreamt that the spirit of the tree appeared to him in his sleep and spoke to him as follows: "What is it you intend to compare me with? Is it with fine-grained wood? Look at the cherry-apple, the pear, the orange, the pumelo, and other fruit bearers? As soon as their fruit ripens they are stripped and treated with indignity. The great boughs are snapped off, the small ones scattered abroad. Thus do these trees by their own value injure their own lives. They cannot fulfil their allotted span of years, but perish prematurely because they destroy themselves for the (admiration of) the world. Thus it is with all things. Moreover, I tried for a long period to be useless. Many times I was in danger of being cut down, but at length I have succeeded, and so have become exceedingly useful to myself. Had I indeed been of use, I should not be able to grow to this height. Moreover, you and I are both created things. Have done then with this criticism of each other. Is a good-for-nothing fellow in imminent danger of death a fit person to talk of a good-for-nothing tree?" When the carpenter Shih awaked and told his dream, his apprentice said, "If the tree aimed at uselessness, how was it that it became a sacred tree?"
"Hush!" replied his master. "Keep quiet. It merely took refuge in the temple to escape from the abuse of those who do not appreciate it. Had it not become sacred, how many would have wanted to cut it down! Moreover, the means it adopts for safety is different from that of others, and to criticize it by ordinary standards would be far wide of the mark."
It's that last line that really draws me in. David Hinton, whose translations I like a lot, renders it this way: "Look it isn't like the rest of us: it's harboring something utterly different. If we praise its practicality, we'll miss the point altogether, won't we?"
This really helped me with Aidan. There he was, and still is, a boy who will never grow beyond infancy, who will never live up to our standards of productivity or accomplishment. But, as Chuang Tzu suggests, even the apparently useless have their value, and it could be a value that we miss completely if we insist on using our usual standards of worth. I've always liked that passage....