Father Joseph-Adrien Lelarge de Lignac was born in Poitiers (France) in the year 1697. He died in Paris in 1762. In 1731, he was ordained priest and he became an oratorian the following year. His Letters to an American are an attempt to refute Buffon and Condillac, this book was prepared in collaboration with Réaumur (J. Roger, P. Charbonnat). Here, we present some elements of the correspondence between Lignac and Réaumur which show a scientific dialogue about 1736-1748 and we try to bring some foundations of the thought of Lignac to light. Between empiricism and metaphysics, Lignac stands up for scientific questions being subject to literal reading of the Bible. But Lignac is also the first one to perceive a way of thinking in Buffon Natural history (T. Hoquet). According to him, the thought of Buffon is a deviation in an order of knowledge, because Natural history would especially be based on observation and experiment. So in this paper, we emphasize two original sources in the thought of Father de Lignac: Malebranche philosophy and his collaboration with Réaumur.