الاجتماع: وهو كون جوهرين بحيث لا يمكن أن يتوسطهما ثالث
Conjunction is two substances being so situated that no third substance can intervene between them.
The decisive condition is the impossibility of an intervening third. Two things being in one room merely shows general proximity; it does not establish technical conjunction.
In the classical atomistic scheme, neighboring indivisible parts are used to illustrate this relation. Technical contiguity must be distinguished from ordinary closeness in order to preserve the logic of the definition.