We can find such a stipulation for example in BOYER, C., 1947, Handboek der Wijsbegeerte, Dutch translation, Volume II, p. 150 :
"One should not imagine that the average qualities originate from the simultaneous presence of the qualities of the elements, having unified themselves into ONE quality : where indeed there are no forms, neither there are the qualities of the forms. But the average quality of the mixtum originates from the form of the mixtum, even though it corresponds to the properties of the elements."BOYER says in fact that one and the same individual Substance, the mixtum (for example a chemical compound), cannot consist of parts that are themselves Substances (this is the stipulation). So the elements of the mixtum are themselves not Substances, implying that no substantial form of any element is present in the mixtum. And because qualities (which are Accidents) are in some way derived from, or at least sustained by, the substantial form, they cannot be present in the absence of the latter. So even the qualities that resemble those of certain elements cannot originate from (the substantial forms of) those elements, but from the substantial form of the mixtum. The resemblance is due to a certain 'genetic' (chemical) kinship between the mixtum and its elements : The mixtum could be made from the elements, and the elements can in principle be 'recovered' from the mixtum (See the Essays on the mixtum : The Mixtum and its Elements and In what way do the Elements exist in the mixtum? ).
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