appears to me open, namely, to attempt nothing beyond a general sketch of the subject.

I shall, therefore, indicate rather than elaborate the line of thought: I shall point out the way leading to the goal, but not follow it thither; in short, I shall present but a very small part of what, under other circumstances, could be adduced. In adopting this attitude for the reasons stated, I wish, before beginning, to emphatically remark, that in any case the actual problem put forward has now been solved; consequently, that what I here add is an opus supererogationis, an appendix to be given and taken entirely at will.