§ 1. The Term, its Objects and Properties
25. The Object of the Term. -- Terms are vocal signs which express objects as they are conceived by the intelligence; they are not the expression of subjective concepts as such, or of things as they are in nature, but of things as the intelligence conceives them; in a word, they designate known objects. "It is through the medium of intellectual conception", says St. Thomas, "that words are related to the representation of things." (Voces referuntur ad res significandas mediante conceptione intellectus) .{1}
The word sun, e. g., does not signify the idea of the sun, but the sun itself. And yet that word does not directly designate the sun as it is in nature. For it was long supposed that the sun was a disc moving around our planet; now this is not the true sun, but only the sun as humanity imagined it before the discoveries of Galileo and Copernicus.
It belongs to psychology to study the nature and functions of language.
{1} Summa Theol., I, q. 13, a. 1.