Thesis in epic poetry is from the Greek meaning putting down. In a metron like dactylic hexameter, the thesis is like putting a foot (or meter) down, and it's the long syllable in the first half of the foot. The opposite is arsis which means (in Greek) "lifting up", which prepares the reader for the next footstep. These terms were used in Homer's Iliad and Odyssey and Virgil's Aeneid.