Matthew Arnold uses the term “touchstone” to symbolically represent lines of truly good poetry by poets like Homer or Dante Alighieri. Historically speaking, a “touchstone” was a piece of jasper that was used to test the purity of gold. In the context of this essay, then, Arnold uses it to embody the idea of comparison, proposing that great works of poetry can act as touchstones that will help readers test other works in order to arrive at the real estimate of the poems in question. If readers always have lines of classic poetry in their head, they can compare those lines with whatever they’re reading and, in this way, determine whether or not what they’re reading is genuinely good or not. Arnold’s use of this term is related to his conviction that excellent poetry is different from inferior poetry (by virtue of its high seriousness), just as gold is different from other elements. Furthermore, the idea of the touchstone underscores just how valuable Arnold thinks great poetry is—so valuable, it seems, that it’s comparable to a material that determines the quality of gold.