This paper explores the role theatre played in the life of Charles Peirce and his second wife, Juliette, from 1884–1888. Peirce became acquainted with playwright and director Steele Mackaye, who trained Juliette in the acting techniques associated with “aesthetic expression,” a movement derived from the work of François Delsarte. I first trace the Peirces’ interactions with Mackaye during this period. The paper then demonstrates affinities between Peirce’s semeiotic account of cognition as mediated through feeling and sensation and the architecture of Delsarte’s system of actor training. The latter employs Delsarte’s semeiotique as an analytical tool for conveying the dramatic character’s inner life. A function of mind, semeiotique intertwines with the functions of life and soul to complete the actor’s task. The affinities between these two accounts of semeiotic emerge from the paper’s analysis of Peirce’s 1888 essay on aesthetic expression, “Trichotomic,” and related passages from A Guess at the Riddle.