“In Foundations of the Theory of Signs (p. 6), the three terms in question were defined follows: pragmatics as the study of "the relation of signs to interpreters", semantics as the study of "the relations of signs to the objects to which the signs are applicable", syntactics as the study of "the formal relations of signs to one another."”

Dr. Théophile de Bordeu, in “Conversation Between D’Alembert and Diderot”
D’Alembert’s Dream (1769)

Source: How Maps Work: Representation, Visualization, and Design (1995), p. 368

On The Algebra of Logic (1885)
Context: If the sign were not related to its object except by the mind thinking of them separately, it would not fulfil the function of a sign at all. Supposing, then, the relation of the sign to its object does not lie in a mental association, there must be a direct dual relation of the sign to its object independent of the mind using the sign. In the second of the three cases just spoken of, this dual relation is not degenerate, and the sign signifies its object solely by virtue of being really connected with it. Of this nature are all natural signs and physical symptoms. I call such a sign an index, a pointing finger being the type of the class.
The index asserts nothing; it only says "There!" It takes hold of our eyes, as it were, and forcibly directs them to a particular object, and there it stops. Demonstrative and relative pronouns are nearly pure indices, because they denote things without describing them; so are the letters on a geometrical diagram, and the subscript numbers which in algebra distinguish one value from another without saying what those values are.

Variant: The full characterization of a language may now be given: A language in the full semiotic sense of the term is any intersubjective set of sign vehicles whose usage is determined by syntactical, semantical, and pragmatical rules.
Source: Writings on the General Theory of Signs, 1971, p. 48; as cited in: Adam Schaff (1962). Introduction to semantics, p. 314

Source: "Foundations of the Theory of Signs," 1938, p. 3

Source: Introduction to semantics, 1962, p. 4

On The Algebra of Logic (1885)
Context: Any character or proposition either concerns one subject, two subjects, or a plurality of subjects. For example, one particle has mass, two particles attract one another, a particle revolves about the line joining two others. A fact concerning two subjects is a dual character or relation; but a relation which is a mere combination of two independent facts concerning the two subjects may be called degenerate, just as two lines are called a degenerate conic. In like manner a plural character or conjoint relation is to be called degenerate if it is a mere compound of dual characters.
A sign is in a conjoint relation to the thing denoted and to the mind. If this triple relation is not of a degenerate species, the sign is related to its object only in consequence of a mental association, and depends upon a habit. Such signs are always abstract and general, because habits are general rules to which the organism has become subjected. They are, for the most part, conventional or arbitrary. They include all general words, the main body of speech, and any mode of conveying a judgment. For the sake of brevity I will call them tokens.

Source: "Foundations of the Theory of Signs," 1938, p. 16; partly cited in: [[Alan MacEachren|MacEachren (1995:235)

[O] : Introduction, 0.4
Semiotics and the Philosophy of Language (1984)
Context: A specific semiotics is, or aims at being, the 'grammar' of a particular sign system, and proves to be successful insofar as it describes a given field of communicative phenomena as ruled by a system of signification. Thus there are 'grammars' of the American Sign Language, of traffic signals, of a playing-card 'matrix' for different games or of a particular game (for instance, poker). These systems can be studied from a syntactic, a semantic, or a pragmatic point of view.