During the Kolinahr ceremony, Spock's given a necklace which represents "pure logic." It's pretty fancy (it had a bunch of geometric bangles on it) and would probably go super-well with Spock's eyebrows.
But he rejects it.
This represents his desire to hold on to his human emotions the Kolinahr ritual seeks to destroy. Still, he isn't ready to abandon logic completely, as symbolized by his decision to take the necklace with him after the ceremonial leaders leave.
Here's a bonus: Spock later uses the phrase "pure logic" to refer to V'Ger. And he uses it negatively. Could this be a hint that Spock is now opposed to the hardcore logic of his Vulcan heritage? Or is there something else going on here?