But we, by a love so much refined That ourselves know not what it is,
Here's the flip side to those shallow lovers. Donne returns (finally!) to himself and his wife, the actual subjects of the poem. He reintroduces them ("we"), but then immediately skips off again. It's another two lines for we get a verb for "we." These extra phrases act almost like royal titles, elevating him and his beloved above the commoners.
Donne is sneaky again in line 17. In line 16, he used the word "elemented" to mean "began." Of course it also reminds us of the physical elements. With the word "refined" here, he very subtly prepares his audience for his next metaphor. But we're not there yet…
At first glance, it's tough to see a real purpose to the easy-to-understand line 18. Everything in the poem seems to have tricky double-meanings, but this one seems like good old embellishment and that's it. May we submit a possibility? The line actually parallels the original metaphor—the earthquake and the motions of the spheres. The motions of the planets and stars, remember, was "innocent," undetected and unknown by anyone. Well, so is their love. It is so refined, so far above this world, that not even the poet himself knows what it is. Line 18 refers all the way back to line twelve to help the whole extended metaphor hold together. That kind of staying power is a sure sign of a conceit.
Inter-assurèd of the mind, Care less eyes, lips and hands to miss.
A couple of the central contrasts of the poem come into play in line 19. First, you've got the contrast between lovers who are only connected by their physical bodies and those who share a spiritual bond. Donne emphasizes that he and his beloved are connected by their minds.
The other central contrast that is introduced here is hidden in that not-so-poetic phrase "inter-assured." Donne claims that he and his wife share their mind and spirit with one another.
It's easy to say that Donne looks down on physical attraction, but that's not quite fair. He is merely stating that, when that physical attraction in the only thing a relationship is based on, it's never enough. He loves his wife, and he will miss her dearly. They just "care less" about missing each other physically than their spiritual connection.
Notice in line 20 that Donne divides up the person into parts ("eyes, lips and hands"). This synecdoche (representing a whole with just a part), reminds us that when our 'love' is only physical, it cheapens the other person and turns them into a commodity.