What did Matthew Arnold mean by using touchstones to judge poetry?

In his essay "The Study of Poetry," Matthew Arnold introduced the concept of "touchstones" as a method for distinguishing genuine, high-quality poetry from inferior or counterfeit work.1

Because Arnold believed that poetry would increasingly serve to interpret, console, and sustain humanity in an age of declining religious certainty, he argued that readers must be able to identify the best literature. To achieve this, he proposed that critics and readers should "steep" themselves in the works of acknowledged masters.1

By using specific passages from these masters as touchstones—or standards of comparison—one can evaluate new or unfamiliar poems based on their "high seriousness" and their superiority of diction and movement. Essentially, these touchstones act as a benchmark of excellence, allowing the reader to measure the quality of other works against the proven aesthetic and moral depth of the greatest poets.1