But in attempting to dive into the mysteries of nature, it is of importance to know if the heavenly bodies act upon each other by impulsion, or by attraction; if a certain subtile invisible matter impels them towards each other ; or if they are endowed with a secret or occult quality, by which they are mutually attracted. <br class="br">"Letters of Euler to a German princess, on different subjects in physics and philosophy", Volume 1 - página 211 http://books.google.com/books?id=_1oIAAAAIAAJ&pg=PA211, de Leonhard Euler, Henry Hunter, Jean-Antoine-Nicolas de Caritat Condorcet, Traduzido por Henry Hunter, Edição 2, Editora Murray and Highley, 1802