This paper is devoted to Kazimierz Twardowski's thesis that the unity of a compound object (a whole) can be ensured only by the relations between its parts and the object itself. Twardowski's idea of unity raises many difficulties, especially the threat of petitio principii: the whole is presupposed as furnishing the ground for the unification of its parts, and yet it also seems to be the result of this unification. To avoid these problems, Edmund Husserl sought to refute Twardowski's thesis, (...) and ascribed the role of a principle of unity to the foundational relationships which directly hold between the parts of a whole. Roman Ingarden then seemed to return to Twardowski's concept of unity, but employed it within a different theory of objects, according to which to be an object is to be a subject of properties. I seek to demonstrate that: (1) Twardowski's thesis is sound if a compound object is something over and above its parts; (2) with respect to Husserl's solution, it is not clear as to whether the whole is really something over and above its parts; (3) Ingarden's conception of the subject‐properties structure saves Twardowski's thesis; (4) Ingarden's theory of higher‐order objects is invalid. (shrink)