Full Idea
A whole must at least approximate to this condition: every member of some division of the object stands in a certain relation to every other member, and no member bears this relation to anything other than members of the division.
Gist of Idea
A whole requires some unique relation which binds together all of the parts
Source
Peter Simons (Parts [1987], 9.2)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.327
A Reaction
Simons proceeds to formalise this, and I suspect that he goes for this definition because (unlike looser ones) it can be formalised. See Simons's Idea 12865. We'll need to know whether these are internal or external relations.
Related Idea
Idea 12865 Analytic philosophers may prefer formal systems because natural language is such mess [Simons]