The article consists of two parts. In the first one (introductory) I recall—following Edmund Husserl, Stanisław Ossowski and Adam Schaff—the main formulations of the “principle of transparency of the sign.” In these formulations it is usually said about (1) the transparency of the sign regarding objects denoted by the sign (denoted, designated and/or named), or (2) the transparency of the sign regarding its meaning (respectively, events, states of affairs and facts designated by the sign). However, as Husserl pointed out, one can also speak about (3) the transparency of the sign in relations to the activities and mental states of the sign’s users (senders and recipients). After all, only due to the transparency of the sign understood in this way, it is possible for people to communicate with each other, thus the sign can also has an expressive and communicative function. In turn, the second part of the article (essential) contains a reconstruction of the Leon Koj’s approach; Koj gave a consistently formalized form to the theory of sign based on the principle of transparency— the form of an axiomatized logical system (using Quine's formalism from his Mathematical Logic). One of Koj's main goals was also to indicate the close relationship between semantics and pragmatics, and even the primacy of pragmatics over semantics. Formal-logical tools have also shown that the theory of sign based on the principle of transparency neither contravene The Law of Non-Contradiction (at least in its psychological formulation), nor contain or imply semantic antinomies such us antinomy of the liar. Because it is a theory easily negotiable with Alfred Tarski’s theory of language levels.