Monsignor Said Poem by Emmanuel George Cefai

I'm not sure about the meaning of this exchange. The Monsignor says poems can be medicinal, which means they work to heal. Then the speaker says, if I'm reading this right, the poet has to make a conscious effort for poems to have this property. But what does I calculate therefore I will mean? The reference to Descartes is unclear to me. You wrote a poem leaving big gaps between each pair of lines. Which means we have to fill in the gaps, and I can't see what that fill-in should be.