Premise: Look for Ghiberti's vigorous bronze of the city's patron St. John (east wall, on the Via de' Caleiuoli); St. Matthew, the bankers' tax-collector turned Apostle (west); Donatello's St. George, the armorers' dragon-killer (north, a bronze cast of a marble original in the Bargello); St. Mark, whose vividly sculpted robes do credit to the linen-drapers he protects (south); and Nanni di Banco's outstanding conspiratorial group of Four Crowned Martyrs for the sculptors' own guild of stonemasons and wood-workers (north).
Hypothesis: Ghiberti was a sculptor. 
Label: entailment