“Foundations of the theory of signs,” published by Charles W. Morris in 1938, deals with the relations between semiotics and science, and those between semiosis and semiotics, among others. Compared with previous research regarding the aspects of semiotics being meta-science, the three dimensions of semiosis, semiotic as organon of the sciences, etc., this article does push forward the development of linguistics and semiotics since the late 1930s. However, its discussions on semiotics being meta-science, the nature and classification of signs, the three dimensions of semiosis, organism in the sign relations, universals and universality of signs, and thing-language are either not logically rigid or inadequate in content and scope. For a piece of work discussing the theoretical foundations of signs, it does not consider sign transformation, a universal and ubiquitous sign activity, which is not consistent with the keyword “foundations” in its title. A critical analysis of these problems involving the aspects mentioned above may not only enrich the visions of triadic sign relations, semiotics, and translation semiotics, but also inspire future semiotic studies and even other new research related to signs.