But side by side with this development and gradual popularisation of the new science of International Law, ideas of a less practical, but not less fruitful kind had been steadily making their way and obtaining a strong hold upon the popular mind. The Decree of Eternal Pacification of 1495 had abolished private war, one of the heavy curses of the Middle Ages. Why should it not be extended to banish warfare between states as well? Gradually one proposal after another was made to attain this end, or, at least, to smooth the way for its future realisation. The first of these in point of time is to be found in a somewhat bare, vague form in Sully’s _Memoirs_,[33] said to have been published in 1634. Half a century later the Quaker William Penn suggested an international tribunal of arbitration in the interests of peace.[34] But it was by the French Abbé St. Pierre that the problem of perpetual peace was fairly introduced into political literature: and this, in an age of cabinet and dynastic wars, while the dreary cost of the war of the Spanish succession was yet unpaid. St. Pierre was the first who really clearly realised and endeavoured to prove that the establishment of a permanent state of peace is not only in the interest of the weaker, but is required by the European society of nations and by the reason of man. From the beginning of the history of humanity, poets and prophets had cherished the “sweet dream” of a peaceful civilisation: it is in the form of a practical project that this idea is new. [33] _Mémoires ou Œconomies Royales D’Estat, Domestiques, Politiques et Militaires de Henri le Grand, par Maximilian de Bethune, Duc de Sully._ [34] See _International Tribunals_ (1899), p. 20 _seq._ Penn’s _Essay towards the Present and Future Peace of Europe_ was written about 1693, but is not included in all editions of his works.