By the Grace of G‑d 11 Tishrei, 5712 Brooklyn, N.Y.
In reply to your letter and questions contained therein:
I. Re the apparent contradiction between the Omniscience of G‑d and man's Free Will (or choice of action), there is a whole literature dealing with the subject,1 and it is impossible to give an outline of it in the course of a letter. I would refer you, at least, to the Rambam (Hilechoth Teshuvah, ch 5).2 However, I cannot leave without some answer, so I will state very briefly:
G‑d's foreknowledge is no contradiction to man's free choice. What could be a contradiction to free choice is compulsion, not knowledge. Hence, a foreknowledge that is not compelling or forcing, is in no way a limitation on one's free choice of action.
I will cite two illustrations. First, assuming that there are men with prophetic knowledge, their forecast concerning certain people does not compel them to act in a certain way and does not rob them of their free choice of action.
Secondly, as you know, G‑d is not subject to time, and the past, present and future is all the same to Him (He Was, Is, and Will be, all in One). It follows, that to G‑d, future is like the past, and as knowledge of a past action is no contradiction to free choice, so is His knowledge of a future action. In other words: G‑d knows the thoughts of man and his - man's - decisions, and such knowledge does not rob man of thinking and deciding how to act.
II. Re your question as to my opinion of the theory of Evolution. You do not mention what Evolution you are referring to. Presumably the evolution of vegetable and animal life.3
My opinion is, as stated in the Torah, that during the Six Days of Creation, G‑d created the Four Kingdoms (minerals, vegetation, animal and man) independently of each other. Our Sages have enlarged upon this question in detail. However, this Creation does not deny the possibility of evolution after that of particular species through various mutations.
III. With regard to your question concerning the role of Aggadah in the Talmud, particularly those dealing with medicine, I want to point out that you are touching upon two distinct questions: Aggadah in the Talmud, and Medicine in the Talmud.
As to Aggadah, not all Aggadoth can be treated equally. In the introductions to various editions of Ein Yaakov you will find how our Sages classify the Aggadoth of the Talmud.
As the question of Medicine in the Talmud, they are not at all as fantastic as they may appear. As a matter of fact, many medical suggestions in the Talmud have been confirmed in recent years as to their therapeutic value, although medical science had long derided them.
Generally speaking, however, inasmuch as the nature of the human organism had undergone many changes since those days, the medical advice contained in the Talmud cannot be applied nowadays. But it is quite certain that in their days the remedies were quite effective.
For references consult: Tosefoth Moed Koton 11a (ד"ה כוורא),4 Kesef-Mishnah, ch. 4 of Hilechoth Deoth, ch.18,5 and the sources mentioned in Sdei-Chamed, vol. of Kololim, under "R," klal 54,6 where it is stated that due to physical and climatic changes, medical treatment and remedies of old no longer hold good generally.
In the History of Medical Science many illustrations are cited as to changes in both in man's susceptibility to disease and treatment, the development of virus attack, new diseases, etc. There is quite a literature on the subject, and there is no need for me to enlarge upon this subject.7
I am surprised that you do not mention in your letter anything about your activities in influencing others to bring them nearer to Torah and Yiddishkeit, which serves also to strengthen one's own convictions.
With blessing,
Notes and sources
1) See Igros Kodesh, vol. 3, Letter 439, 442, where this subject is treated at great length and a wealth of sources on this subject are referred to. See also letter dated 25 Adar, 5721.
2) For an English rendition of this chapter, see here.
3) For more on the subject of evolution in the Rebbe's opinion see A. Gotfryd (ed.), Mind Over Matter: The Lubavitcher Rebbe on Science, Technology, and Medicine (F.R.E.E. Publishing House, 2015), p. 27, ff. Letters dated 18 Teves, 5722; 17 Cheshvan, 5723.
4) "שמא נשתנו כמו הרפואות שבש"ס שאינן טובות בזמן הזה". Translation: Perhaps [the health benefits of those things] changed similar to the medicines in the Talmud which are no longer effective.
5) "רפואת והנהגת מלכות בבל שבה היו חכמי הגמרא משונה משאר ארצות כדאשכחן שאמרו בסוף פירקא קמא דמועד קטן (דף י''א ע''ש בתוס') שרפואת הדג לשתות עליו מים ובשאר ארצות דרך הרפואה שלא לשתות עליו מים". The medicine and conduct of the Babylonians, the nationality of the Sages of the Talmud, is different than that of other nationalities. As we find in Moed Katan (11a), the healthy way is to drink after eating fish, whereas in other nationalities the healthy way is not to drink water afterward.
6) Accessible here.
7) See also Likkutei Sichos, vol. 23, p. 33, ff.
Source: Secretarial carbon copy.
Published: Teshura, (Mochkin, 30 Sivan, 5758).
Source & Link: צדי"ק למלך חלק ז, pg. 10 [מהצד האנגלי]: https://drive.google.com/file/d/1ETi_bDX3MDh_jMlbM0zTtVlg1CVK5JfC/view?usp=drivesdk Alternative Source: https://www.chabad.org/therebbe/letters/default_cdo/aid/1916345/jewish/Addressing-Problems-of-Belief.htm Mind Over Matter https://drive.google.com/file/d/1zBk2xmWzz9KGYk9Ap9AJ1MViUPzbPthD/view?usp=sharing אס"ל ח"א (הוצ' שני') https://docs.google.com/document/d/1Q5Hp_Rj7FvADXoOgZvggNT8NrQ3_QUM81--XoghUih4/edit?usp=drivesdk https://drive.google.com/file/d/1eXnzw2H42CjR42Xd2ElzJArvE7Wu9yeZ/view?usp=sharing https://drive.google.com/file/d/1xck_ipMugFZUZR-BGYzjovopOz7gh79N/view?usp=sharing https://drive.google.com/file/d/18hIr1a73tp1Xxc_eUjbDJCk2eTSwxbKL/view?usp=sharing
An exerpt in the LAS 6 [Funny that Hagaha is included] https://drive.google.com/file/d/1Dt5sT6-BfCb2wb98njgl-CDSADMDOAuh/view?usp=sharing