Eclecticism is a conceptual approach that does not hold rigidly to a single paradigm or set of assumptions, but instead draws upon multiple theories, styles, or ideas to gain complementary insights in
The grand foyer of the Palais Garnier, by Charles Garnier, 1860–1875. Stylistically, it aimed for a Baroque opulence through lavishly decorated monumental structures that evoked Louis XIV's Versailles. However, it was not just a revival of the Baroque, being more of a synthesis of Classicist styles, like Renaissance, Baroque, Rococo, Neoclassicism etc. Thus, it is an example of eclecticism in architecture.
Early Romanian Revival house on Strada Grigore Alexandrescu in Bucharest, Romania, unknown architect, c. 1900, that mixes Beaux-Arts and Romanian Revival elements and proportions
Building no. 45 on Rue de Courcelles in Paris, unknown architect, unknown date, an example of 19th century architecture that can be called "Eclectic" due to the fact that it uses elements from multiple Classicist styles, like the French Baroque and the Louis XVI style
Posidonius (c. 135 – c. 51 BC), who followed Stoicism, but also the works of Plato, Aristotle and Greco-Roman natural philosophers.