This article is a response to Aleš Chalupa and Tomáš Glomb's article "The Third Symbol of the Miles Grade on the Floor Mosaic of the Felicissimus Mithraeum in Ostia: A New Interpretation". Their interpretation is viewed from a theoretical perspective. Charles Sanders Peirce's theory of signs is applied not only to the historical evidence but mainly to the authors' interpretive attempt. The term "sign" is suggested as more accurate than the term "symbol". Thus, Chalupa and Glomb's interpretation of the third sign of the Miles grade, as it is displayed on the Felicissimus mosaic, might be structured according to the ascent from the iconic to the indexical, and from the indexical to the symbolic interpretive level. It is suggested that an appropriate theoretical framework might support their interpretation and surmount the weaknesses of their argumentation.