Pickup is a beautiful art and each neg is like a stroke of a paintbrush
Just think of a musician. He is a music artist. Music is an art. Like Superunknown said, he must learn technical details such as chords and music theory; it's much like learning a "science" of how to make shit sound good to the human ear.
A PUA learns some technical shit like background science on evolutionary psychology and stuff like that, but, like the musician, his ultimate destination is a place where he can fully express himself in order to share his emotions (in pickup's case, arousal) and to elicit specific responses. It is an art because the emphasis is placed on the expression and how effective it is. It is not so much a science itself because our practices in this community don't exactly follow the empirical standards of the rest of the scientific community.
Science and art, however, are normally difficult to distinguish between one another in the first place, anyway. If you look up definitions for art, it'll be hard to differentiate it from how anyone could define science. The best differentiation that anyone could probably come up with is that art is more "emotional" and that science is more "logical."
Pickup pretty much looks at emotion in a logical sense, but here's something I can offer to clear some confusion:
Newbies who are learning pickup usually make the mistake of over-thinking emotions and therefore limit their own emotional expression when they're sarging. They are trying to be scientists, but it just doesn't fucking work. An artist, however, stays in the moment with his expression and FEELS. He's defining what it's like to be human through his emotions. The "PUA" who tries to be a scientist fails, and the PUA who actually becomes an artist succeeds.
Pickup is an art.