 Arnauld argues at length that this theory of ideas was also held by Descartes (Ideas, 26ff.). This claim has puzzled philosophers from Thomas Reid to the present (Reid 1785, 169). A good example of the more common interpretation of Descartes’ theory is provided by Ian Hacking, who also attributes the theory, as he understands it, to the Port-Royal Logic. Hacking begins with the first sentence of the First Part of the Logic: “We have no knowledge of what is outside us except by the mediation of the ideas within us,” and continues,