The debate on the (in)applicability of Article 2(4) to cyber attacks shows attempts to apply the 1945 prohibition to a 21st-century phenomenon. This article argues that the difficulties encountered in doing so do not result from the particular nature of war in cyberspace but from a paradox that lies at the heart of the prohibition on the use of force. This paradox results from alternately using means and effects as a standard to establish the current scope of the prohibition. Article 2(4) includes the use of minor military force, the use of biological and chemical weapons as well as non-military physical force, yet excludes economic coercion and, possibly, minor cyber attacks. On closer consideration the in-or exclusion of these categories from the prohibition cannot be argued on the basis of the same rule of thumb. In other words, the two standards-means and effects-are mutually exclusive as they lead to different legal qualifications for similar cases. At the same time both standards are needed to come to that which is 'generally accepted' with regard to Article 2(4). As there is no way out of this paradox old debates resurge, current ones are insolvable and those that are yet to be held are bound to display similar patterns. Journal of Conflict & Security Law Oxford University Press 2014.