18 March 1889
Mr Valentine Hynes Esquire
Your letter of sixth instant duly received today. The subject seems to be some political discussion which I do not understand and can therefore offer no opinion upon.
The terms theorem and theory are often used as if they meant the same thing, which is of course not quite the fact, theorem means some proposition to be proved by a chain of reasoning, while theory means the results of such a process of reasoning. Now Sir George Airy propounds a [??] theorem which he proceeds to demonstrate but [??] come upon him before the work was completed and it is still a theorem. But in the present it is constantly referred to “Airy’s Lance Theory”, and the fact of the demonstration is not complete is not held to [??] the use of the word theory; because no man every demonstrated perfectly a theory. Now great effort is always a stepping stone upon which another man can rise to improve the demonstration. The Lunar Theory is the most difficult object in astronomy, and many astronomers have proposed [??] ones. The great Professor Adams of Cambridge, Professor [??] of Gotha: M. [??] of Paris and Sir George Airy and many others, and it is still speaking from an astronomer’s point of view, very incomplete.
H C Russell