The modes expounded by the ancient sceptics became a point of reference for those who try to prove, reach any truth or justify any belief. Due to it, this paper analyses the objections of the sceptic philosophers later to Aenesidemus and previous to Sextus Empiricus. Such objections are generally attributed to Agrippa and they imply a questioning of the grounds of all knowledge that must be followed in order to make a proof or justification. In addition, we will try to point out some of the possible solutions, especially those developed previously by Aristotle.