The text of the letter appears on the site of the Japanese Bertrand Russell Society, https://russell-j.com/cool/L001.HTM. The earliest known version is at https://russell-j.com/cool/TUTIDA-K.HTM.

SS Empress of Asia, Aug. 3, 1921

Dear Sir,
I must regret that during my brief stay in Japan I had no time to answer your kind letter. I will attempt to do so now.

1. With regard to sense-data and sensation, I have somewhat modified my position, as you see from my The Analysis of Mind, now on sale at Maruzen in Tokyo. I do not distinguish two words, one 'real' and the other 'ideal'. I think there is only the 'real' world.
2. Sense-data are defined as those sensibilia of what we are aware. Presumably there are innumerable sensibilia of which we are not aware, but by definition we can have no empirical evidence of their existence.
3. I do not believe in a dualism of matter and spirit. Both matter and spirit are to my thinking, logical constructions not existing things. This point also you will find explained in The Analysis of Mind.
4. I think there are no logical grounds for either asserting or denying the existence of an external world. Psychology only explains why, in fact, most people do assert it.
5. I think very likely your criticism of guild socialism may be sound in theory, but in practice I do not know of any better system suitable for industrially backward countries such as Russia and China.
6. I do not believe in the 'general will'. I think it is a fiction. In philosophy I am an atomist. Even the individual seems to be already a complete logical structure. I think all government is essentially the imposition of the will of the strong upon the weak.

Yours faithfully
Bertrand Russell