 The correspondence between Elisabeth and Descartes begins with Elisabeth’s asking probing questions about how Descartes can explain the ability of an immaterial substance to act on a material substance. At issue in this initial query is the kind of causation operating between mind and body. As Elisabeth frames the issues, existing accounts tie causal efficacy to extension, and in this regard it is significant that she poses her question about the mind’s ability to act on the body, and not the body’s ability to affect the mind. To account for the causal efficacy of an immaterial mind, Elisabeth suggests that Descartes can articulate either the account of causation proper to mind-body interaction or the substantial nature of the mind such that existing accounts could explain its actions. Descartes’s response is not only evasive but opens up further issues, in particular about whether the mind-body union is a third substance, insofar as he appeals to the Scholastic notion of heaviness to address Elisabeth’s concerns (Garber 1983), and intimates there is a contradiction in thinking of mind and body as both two distinct substances and as united (Mattern 1978). In addition, in his responses, Descartes jumps between the two separate issues of mind-body and body-mind interaction (Rozemond 1999). My concern in this entry is not, however, to articulate the views expressed in Descartes’s side of the correspondence.