On the Problem of Business Cycles
There are as many causes for fluctuations in economic life as there are external conditions of economizing. Each of these conditions can change and thereby effect a change in the course of the economic process. If the economy is affected to a greater extent, if farther reaching changes prove necessary and, in particular, if a lasting change in an external determinant of the economy causes disruptions in the economy for a longer period of time, then one can speak of an economic crisis.