SIR,--Your favor of December 17th, has been duly received, and with it the pamphlet on the cause, seat and cure of diseases, for which be pleased to accept my thanks. The commencement which you propose by the natural history of the diseases of the human body, is a very interesting one, and will certainly be the best foundation for whatever relates to their cure. While surgery is seated in the temple of the exact sciences, medicine has scarcely entered its threshold. Her theories have passed in such rapid succession as to prove the insufficiency of all, and their fatal errors are recorded in the necrology of man. For some forms of disease, well known and well defined, she has found substances which will restore order to the human system, and it is to be hoped that observation and experience will add to their number. But a great mass of diseases remain undistinguished and unknown, exposed to the random shot of the theory of the day. If on this chaos you can throw such a beam of light as your celebrated brother has done on the sources of animal heat, you will, like him, render great service to mankind. The fate of England, I think with you, is nearly decided, and the present form of her existence is drawing to a close. The ground, the houses, the men will remain; but in what new form they will revive and stand among nations, is beyond the reach of human foresight. We hope it may be one of which the predatory principle may not be the essential characteristic. If her transformation shall replace her under the laws of moral order, it is for the general interest that she should still be a sensible and independent weight in the scale of nations, and be able to contribute, when a favorable moment presents itself, to reduce under the same order, her great rival in flagitiousness. We especially ought to pray that the powers of Europe may be so poised and counterpoised among themselves, that their own safety may require the presence of all their force at home, leaving the other quarters of the globe in undisturbed tranquillity. When our strength will permit us to give the law of our hemisphere, it should be that the meridian of the mid-Atlantic should be the line of demarkation between war and peace, on this side of which no act of hostility should be committed, and the lion and the lamb lie down in peace together. I am particularly thankful for the kind expressions of your letter towards myself, and tender you in return my best wishes and the assurances of my great respect and esteem. TO MR. THOMAS PULLY. MONTICELLO, January 8, 1812.