Tom weighs in on the question, “is the One Ring sentient?” with some evidence that the answer is “no”. The nerds on Reddit had an interesting discussion about the post. (Sturgeon’s Law applies, of course.) The gang raised a good question about the wheel of fire talking to Gollum, for instance.
It’s tricky, because the word “sentient” isn’t often used according to its dictionary meaning. It means “sensing the world around it”, but people generally use it to mean “thinking”. Until we get to Mordor and the hallucinations start, only the former seems relevant.
A good lens through which to look at Tolkien is to look for real-world analogues of what we see the Ring doing. Fact: It changes size, to get away from its current bearer. Tolkien is careful to say “seems to” all the time, but the physical evidence seems clear. The Ring came off Isildur’s hand, and Gollum’s hand, and it tried to escape from Bilbo several times, and boy did it hate being anywhere near Bombadil! It grew as big as it could in a vain attempt to get out of his palm.
So in some way the Ring knows when it’s not going to get anywhere with its current bearer. It knows when an opportunity for something more congenial comes around. And it can change its shape accordingly. Is there a real-world analogue for this?
Seeds have one. They can sense moisture, temperature, and gravitational potential as gradients around them. When they get the combination of moisture decreasing, temperature increasing, and gravitational potential increasing all in the same direction, they sprout, and send a shoot that direction to get out of the dirt and into the sunshine. This is parallel to what the Ring is doing, if we can find some field around it that relates to Sauron’s power instead of earth and water. Some kind of luminiferous aether, except for the power of the Ainur. Ilmen, perhaps? Could Sauron or the Nazgul be distorting the density of ilmen as part of their attempts to draw the Ring to them? (Of course, the palantir can do something similar, so maybe it’s something more down-to-arda than that.)
Anyway, the gradient of the whatever-field affects the Ring’s size. When it’s near a person more congenial to Sauron’s goals, which could be someone more powerful and closer to evil, or someone less powerful but less good, it expands. This ties in with the complexity of Gollum’s character — he’s neither good nor evil, not really anything except lust for the Ring, so any random goblin would be a better host, and off the Ring fell.
P.S. Anyone who doesn’t like the idea of Sauron creating a thing with the power of a seed (as I’m sure JRRT wouldn’t) is invited to use a slime mold as the model instead.