During his lifetime, Henri Poincare published three major philosophical books which achieved great success: La Science et L'hypothese (1902), La Valeur de la Science (1905) and Science et Methode (1908). After Poincare’s death, a fourth volume of his philosophical works was published by his heirs as Dernieres Pensees (1913). Around 1919, Gustave Le Bon wrote to Poincare’s widow. As the director of the Bibliotheque de Philosophie Scientifique at Flammarion, he asked her permission to publish a second posthumous volume. L'Opportunisme Scientifique was intended to be the fifth and final volume of Poincare’s writings. Louis Rougier had elaborated the project, with the collaboration of Gustave Le Bon and the approval of the philosopher Emile Boutroux and his son Pierre. However, because of the reservations of the mathematician’s heirs, this book was never published and Dernieres Pensees remained his last philosophical book. This is the first publication of the fifth and final volume of Henri Poincare’s philosophical writings. In addition to publishing the late mathematician’s writings, the book also aims to throw some light on the work’s history and the early posterity of Poincare’s philosophical thinking.