Whereas those classified as "early dialogues" often conclude in aporia, the so-called "middle dialogues" provide more clearly stated positive teachings that are often ascribed to Plato such as the theory of Forms. The remaining dialogues are classified as "late" and are generally agreed to be difficult and challenging pieces of philosophy. This grouping is the only one proven by stylometric analysis. Among those who classify the dialogues into periods of composition, Socrates figures in all of the "early dialogues" and they are considered the most faithful representations of the historical Socrates.