  In 1966 Tarski does not propose a definition of the concept of logical constant. One such definition, based on the 1966 idea, appears in a book Tarski wrote in collaboration with Steven Givant, published in 1987, four years after Tarski’s death. The main concern of Tarski and Givant (1987) is to indicate how to develop set theory in several different languages and to compare their expressive power. They define the concept of logical constant for constants in the vocabulary of a certain class of languages—all of which are in a certain sense extensions of a basic language in which they are particularly interested, weaker than the languages in which set theory is usually developed in means of expression, and yet sufficiently powerful for many purposes. The vocabulary of the basic language is composed of three two-place predicate constants of the second type and four two-place function constants of the third type; the vocabulary of the extensions considered by Tarski and Givant can only include constants of those two kinds. (However, as we will see, there does not seem to be any problem in extending the range of applicability of their definition to wider classes of languages.) They then prove several results about that class of languages, results that involve the defined concept of logical constant.