The problem of establishing a perfect civic constitution is dependent upon the problem of a lawful external relation among states and cannot be solved without a solution of the latter problem. Immanuel Kant
It is once again the vexing problem of identity within variety; without a solution to this disturbing problem there can be no system, no classification. Roman Jakobson
The UK's decision on Brexit has caused the problem and the UK will have to solve it. Without a solution there will be no withdrawal agreement and no transition. Donald Tusk