Full Idea
A whole must possess an attribute peculiar to and characteristic of it as a whole; there must be a characteristic relation of dependence between the parts; and the whole must have some structure which gives it characteristics.
Gist of Idea
A whole must have one characteristic, an internal relation, and a structure
Source
Rescher,N/Oppenheim,P (Logical Analysis of Gestalt Concepts [1955], p.90), quoted by Peter Simons - Parts 9.2
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.334
A Reaction
Simons says these are basically sensible conditions, and tries to fill them out. They seem a pretty good start, and I must resist the temptation to rush to borderline cases.