For me, philosophically, UL came from the inside out, in a peculiar kind of way:
" ...Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us, looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God." (From the beginning of the 12th Chapter of the book of Hebrews in the Bible)
As it plays out in my life, "UL" and "Leave No Trace" backpacking (in approach and practice) are natural extensions of the philosophy that I live by, which is contained in or deduced from the Bible, which is in turn probably best summarized in the Westminster Confession of Faith and Catechisms.
From classic 17th century literature, John Bunyan's A Pilgrim's Progress, begins the complementary allegorical backpacking scene quite appropriately:
"As I walked through the wilderness of this world, I lighted on a
certain place where was a den,  and laid me down in that place to
sleep; and as I slept, I dreamed a dream. I dreamed, and behold, I saw
a man clothed with rags, standing in a certain place, with his face
from his own house, a book in his hand, and a great burden upon his
and later on in the book...
"and the very sight of
him made my burden fall off my back; for I groaned under a very heavy
burden, but then it fell down from off me."
In a nutshell, Bunyan again says it well:
"Up this way, therefore, did burdened Christian
run, but not without great difficulty, because of the load on his back.
He ran thus till he came at a place somewhat ascending; and upon that
place stood a cross, and a little below, in the bottom, a sepulchre. So
I saw in my dream, that just as Christian came up with the cross, his
burden loosed from off his shoulders, and fell from off his back, and
began to tumble, and so continued to do till it came to the mouth of
the sepulchre, where it fell in, and I saw it no more.
Then was Christian glad and lightsome, and said with a merry heart, "He
hath given me rest by his sorrow, and life by his death." Then he stood
still a while, to look and wonder; for it was very surprising to him
that the sight of the cross should thus ease him of his burden. He
looked, therefore, and looked again, even till the springs that were in
his head sent the waters down his cheeks."