 A “philosophical language” is one “without any deficiency, superfluity, or equivocation” (OM 1, prop. 83), and what makes sight a “philosophical language” for feeling is that the two match each other: “The same qualities are made by means of light to impress vibrations on our eyes, which correspond in great measure to those made on the sense of feeling, so as to vary with their varieties” (OM 1, prop. 30). Feeling and vision flow in tandem, with no missing, extraneous, or equivocal moves. It is thus joint impression, the correlation of the flows of sensation through two (or more) sensory modalities, so that the sensations one “vary with the varieties” of another, that accounts for the fact that we have ideas at all.