By the Grace of G‑d
18th of Teveth, 5720 [January 18, 1960]
Brooklyn, NY
Greetings and Blessings:
I received your letter of January 10th, in which you ask my explanation of the reference of the four basic elements (Yesodoth) mentioned in chapter one of the Tanya, and you ask me how it is possible to reconcile this with modern chemistry which recognizes over one hundred elements.
Prefatorily, I must make at least two corrections in your letter. One, the origin of that statement in the Tanya, is not as you write, but it is to be found in the Medrash Rabba Bamidbar 14:12, and at greater length and in greater detail in many parts of the Zohar, and further explained in other books of the Kabbalah. Two, modern chemistry does not recognize over one hundred basic elements but a considerably fewer number if matter is to be reduced to it's basic components or particles. (For the so called elements are themselves made up of atoms, which are the smallest particles into which an element can be divided and yet retain its properties and characteristics, but the atoms themselves are further made up of further smaller particles), such as electrons, protons, neutrons.
Thus the answer to your question lies in the proper definition of the terms under discussion. For as indicated above, the so called element is not the basic particle matter. Even the term atom which originally meant something indivisible, is an archaism now employed only for conveniences, as it no longer corresponds to it's original meaning. Similarly when we speak of an individual as being an element of society this does not mean that the individual himself is not composite.
This should be borne in mind when we consider the tern Yesodot in the Zohar, Midrash Rabba, Kabbalah, etc., and, of course, in the Tanya and other Chabad sources. This does not mean something which under normal circumstances are indivisible or unchangeable, but the actual so-called "bricks" or components which make up everything that exists in the world. I might also mention that there is another school that conceives these four Yesodot, not in their physical aspects but rather qualitatively, that is to say, "fire," in the sense of the properties of hot and dryness; "water," in the sense of coolness and humidity, and so on.
I hope that this will answer your question, if you have any further questions do not hesitate to write again.
With all good wishes, and with blessing,
Re: Yesodot in Bamidbar If you read the full citation it is "Medrash Rabba Bamidbar 14:12" i.e. it is a citation to the Midrash ON the book of numbers\Bamidbar but not the book of numbers itself. Part of the confusion is our fault as our system automatically hyperlinked to "Bamidbar 14:12" when it reads those words (missing out the first part of the citation). It seems that computers still have ways to go...