What are the two qualities Gardner says make up a true artist?

American novelist John Gardner (1933–1982) is best known for his 1971 novel Grendel, a retelling of the medieval epic poem Beowulf, which won the 1976 National Book Critics Circle Award. Commenting on human creativity, Gardner explained that

The true artist plays mad with his soul, labors at the very lip of the volcano, but remembers and clings to his purpose, which is as strong as the dream. He is not someone possessed, like Cassandra, but a passionate, easily tempted explorer who fully intends to get home again, like Odysseus.

Breaking Gardner's quote down into pieces, we can see that he identifies two key qualities of the "true artist." They could be summarized as (a) intensity and (b) focus. The artist is intense in his wide-ranging and unbounded creative exploration. At the same time, he is not without direction, remaining attentive to his ultimate goal.
Below, I've linked to an audio review on NPR, which does not address this quote specifically but provides a good summary of Gardner's creative perspective and helps contextualize his view.