In “On the Simplicity of the Soul” (1991), Chisholm pursues this Leibnizian line. He identifies a sort of property that he characterizes as “qualitative”. Any such property would characterize substances (rather than other properties, or states, or abstract objects); it would be a property that could characterize a simple thing; it would be an internal (or, as some might prefer to say, “intrinsic”) property of anything it characterized -- thus a thing existing in isolation could have such a qualitative property. Chisholm says that the familiar mental properties that we know ourselves to have (e.g., the property of hoping for rain) are in this way qualitative. So we know that we have qualitative properties. But, as he says, we never know, with respect to any compound thing, that it has any such qualitative property. Thus, there is reason to doubt that we are compound things. We know ourselves to have properties of a kind such that we do not know, of any compound thing, that it has properties of that kind. It’s not entirely clear that this argument establishes that people are not compound things. Perhaps it shows only that for all we know we might be simple entities.