His knowledge was manifested in a multitude of essays that he grouped in the eight volumes of "The universal critical theater" (1727–1739) and in the five of "Erudite and curious letters" (1742–1760). Feijoo saw the necessity of writing to move Spain away from its delay; with this intention, he gave a didactic character to his work, noticeably catholic, but with the intention that the new European currents penetrated, at least, in the illustrated classes. He was very critical with the superstitions and the false miracles.