There is a letter from the Rebbe to Rabbi Yerachmiel Benjaminson of London, written in Paris in 1939. The letter talks about tzimtzum. I quote (changing some of the terms back into the presumed original):
"The crux of the differences centers around two issues: Should the concept of tzimtzum be understood literally k'pshuto or not, i.e., are we speaking about a withdrawal of the light, or merely its concealment? Did the tzimtzum affect merely Or, G-d’s light, or did it affect also the Source of light Maor? It is possible to outline four different approaches:
1)
tzimtzum k'pshuto, and moreover, that it affected
Maor.
2)
tzimtzum k'pshuto, but that it affected only
Or.
3)
tzimtzum eino k'pshuto, but it affected the
Maor as well.
4)
tzimtzum eino k'pshuto, and it affected only
Or.
The misnagdim at the time of the Alter Rebbe followed the first approach mentioned. They explained the expression “There is no place apart from Him,” meaning — apart from His providence. They said, moreover, that the approach which states that G-d’s essence is found everywhere contradicts the laws applying to [the restrictions against prayer and Torah study] in places of filth, as reflected in the notices and proclamations which were circulated at the time of the Baal Shem Tov and the Alter Rebbe.
Reb Chayim Voloziner the author of Nefesh HaChayim follows the third approach mentioned above. In this, he differs from his master, the GRO.
[As chassidim,] we follow solely the fourth approach mentioned which explains that the concept of tzimtzum eino k'pshuto, and that it affects only Or, but not Maor. [Indeed, within the light,] it affects only the lowest level of the light which existed before the tzimtzum, as explained in Chassidus."
There are worthwhile contentions to the letter by Simcha Coffer published on the Avodah Torah Discussion Group:
"1) First of all, he [LR] implies that Tzimtzum in Or and eino kipshuto is a
Chassidishe approach whereas I believe that none of the gedoley Yisrael on the two sides of the Chassidim versus Misnagdim debate believed differently. In fact, the problem R' Chaim (Volozhiner) had with the Chassidim was this very issue; he understood from the 'hamon am' that they understood Tzimtzum to be in Maor as opposed to Or (but eino kipshuto) which is why they were not nizhar regarding tefilla and limud haTorah in mikomos mitunafim (because since the Tzimtzum is eino kipshuto, the tinuf is really just another manifestation of Hashem's essence chs'v) or like the Gra characterized it in one of his bans "ha'omer l'eitz Avi atah".
2) He states that the Gra understood Tzimtzum to be in Maor. IMO, this is a grave error. The Gra would never say such a thing chs'v. In fact, in the likutim at the back of his pirush on Safra d'tzniyusa, he says exactly the opposite. (See Sod haTzimtzum pg. 138 in Bezalel Naor's edition of Safra di'Tzniyusa). Obviously the LR did not see this sefer or he would have never said such a thing.
3) He states that R' Chaim's shita was that it was eino kipshuto but in
Maor. This too is impossible to say and is contradicted several times by a close reading of Nefesh haChaim v'ein kan makom l'ha'arich.
4) He states that R' Chaim argued on his Rebbi in this sugya, an assertion which I consider wholly untenable.
The crowd on Avodah is of the highest quality but I keep on entertaining the notion that we may be violating the prohibition of Kvod Elokim Haster Davar by discussing these musagim in public."
