It may be that you will not only agree with all I have said, but have already said much of it yourself. But there are some passages in your _Fundamental Problems_ which seem to imply otherwise. I think the great objection I have to urge against Kant, and also perhaps against you, is that you do not distinguish as clearly as I could wish between symbolic argument and real, though subjective, knowledge. And the only way to distinguish between them is by inquiring into the definitions of the terms. For example, on p. 165 of _Fundamental Problems_ you say that to four-dimensional beings Kepler’s third law “would most probably appear as ‘the cubes of their times of revolution being proportional to their mean distances to the fourth power.’” Now what sort of assertion do you take Kepler’s law to be? Originally it was a purely empirical law obtained by pure induction. If the four-dimensional people obtained their law the same way why should the result appear different to them? Or do you conceive the law to be deduced from Newton’s theory of gravitation? But even so the law of the inverse square was obtained empirically. If you think that law can be explained (as the analogous law for the distribution of light can) by the supposition that the integral of the force over all points at a given distance from the origin is constant, still this supposition is purely gratuitous unless established by induction from experience. If you grant any one of these suppositions you can by symbolic argument obtain the law corresponding to Kepler’s for a four-dimensional space. But I may mention that in no case does the result you anticipate come out. On the first two suppositions the law would be unaltered. On the last supposition the law of gravity would be changed to the inverse cube; but after that the solution of the problem has nothing to do with four dimensions—it is a two-dimensional problem only. The result is that in general planets could not move in closed orbits at all. They might conceivably revolve in circles, but such a condition would be unstable, and if it obtained their periodic times would vary as the squares of their distances. Again you say (p. 74) “the doctrine of the ‘conservation of matter and energy,’ although it has been discovered with the assistance of experience, can be proved in its full scope by pure reason alone.” I should very much like to see your proof (which I cannot find in _Fundamental Problems_). How do you define the terms of the doctrine? Do you deduce the proof from these definitions—that is do you make it a truism? Or do you base it upon subjective axioms as I do my geometry? Or if you base it on objective facts, how do you prove those facts by pure reason alone? And if it is purely a subjective proof, how, can you say the doctrine is proved “in its full scope”? Surely objective applications come within its scope?