A spark they strike only to let it go out sometime later (11)
This line shows the speaker's views of his life, and perhaps all reality, as transient. It's just a little flame in the darkness, insignificant, let to burn out. An interesting way that this line reflects on reality is the word "they." It's as if the power that decides when we live and die, thought by many people in the word to be one being, God, is suddenly plural, a "they." Stretch your mind and think about some great fire lighting committee that decides when we live and when we die—it's not a very heartening version of reality.
Essentially everything visible is artificial (14)
This line smacks "reality" on the head. Many people think that "seeing is believing," but our speaker here believes that whatever we see isn't real at all, but some artificial construct. We could view this line as stating the belief that whatever's beneath the visible is the most important, but somehow, we get the sense that it's not that optimistic. Knowing our speaker, this line is expressing his doubt in the reality of the world that surrounds him.
Despite the fact that I warm up wherever your hand touches me (16)
This line holds up touch—the human touch of the speaker's lover—as one of the things that is most real about the world. In this context, the speaker is denying its reality, but the details he provides as proof that the world is unreal actually have the opposite effect. They show us that the speaker doubts his own doubts about the reality of the world. So much doubt—it makes our head spin just to read about it! Now just imagine how the speaker must feel.
I see your face but lack all faith in it (18)
Now we can see that the speaker's doubts are ruining things for him. Here's a woman, whom we can assume from the rest of the poem that our speaker is in love with, and he sees her face. Yet because he's unsure about the reality of the world, he can't believe in her. And it's hard to love someone you can hardly believe in. There is, however, another view to take in this line. It's possible that the speaker's faith in his lover and the reality of the world has been shaken by the intensity of his passion for her. It's not possible, perhaps, for such passion to exist in the version of reality held by most people in the world.
The sun and your heart are compacted of the same substance (25)
This line is a great example of a Surrealist version of reality. Sure, our speaker probably doesn't think that this is literally possible, but a big part of Surrealism is using imagery that stretches the realms of the real, of the possible. When crazy, wild things can become real in words and dreams, the reality of what we see everyday fades a little.