According to the provisions laid down in Article 19 of the 1969 Vienna Convention on the Law of Treaties a state that consents to be bound by a treaty may not formulate such reservations to the agreement, which are incompatible with its object and purpose. This socalled 'object and purpose criterion' has long been puzzling actors of public international law. What does it mean for a reservation to be incompatible with 'the object and purpose' of a treaty? The answer suggested below is the following: a state may not formulate a reservation, if it means (i) that an application of the treaty as modified would run counter to a telos of the treaty, or (ii) that a remaining part of the treaty would be emptied of practical meaning.