 Thus, if we wish to solve some problem, we should first of all consider it solved, and give names to all the lines—the unknown ones as well as the others—which seem necessary in order to construct it. Then, without considering any difference between the known and the unknown lines, we should go through the problem in the order which most naturally shows the mutual dependency between these lines, until we have found a means of expressing a single quantity in two ways. This will be called an equation, for the terms of one of the two ways [of expressing the quantity] are equal to those of the other. (AT 6: 372, MOGM: 179)