 Arnauld adds that he continues to have difficulty with Leibniz’s account of “the possibility of things” and his conception of God as choosing the actual world out of an infinity of possible worlds, a point on which Leibniz agreed with Malebranche, and he asks Leibniz to clarify his “hypothesis of concomitance or agreement between substances,” and his statement that if a material thing is not merely an appearance, like a rainbow, or an accidental aggregation of parts, like a pile of stones, it must have a “substantial form.” Leibniz had mentioned both of these points in his immediately preceding letter (LA, 64–66, 58). According to the hypothesis of concomitance no created substance ever acts upon any other created substance. But every later state of any given created substance is caused by an earlier state of the same substance, and the histories of the individuals are coordinated by God so that they fit together into the history of the actual world. Leibniz also says that one substance, x, may “express” another substance, y, more distinctly than y expresses x, and in that case, one may say that x causes changes to occur in y. He uses this position to explain the relation of mind and body. Arnauld asked him to explain his view further by applying it to the example of a man who feels pain when his arm is wounded, and the example of a man wants to take off his hat and raises his arm. Arnauld also raises seven objections to Leibniz’s claims about substantial forms (LA, 66–67) and defends the Cartesian view that extension is the real nature of matter, which he takes to imply that bodies have only varying degrees of “improper unity” (LA, 88).