Our Rabbis taught: Five things make one forget one's studies: (i) eating something from which a mouse or a cat has eaten, (ii) eating the heart of an animal, (iii) regularly eating olives olives, (iv) drinking water that was used for washing, and (v) washing one foot above the other...
Forgetting one's learning is a subject that also features in Pirkei Avot 3:10, where Rabbi Dosta'i beRabbi Yannai used to say in the name of Rabbi Meir:
Anyone who forgets even a single word of this learning, the Torah considers it as if he had forfeited his life. As it states "Only be watchful and take care for your soul, lest you forget the things that your eyes have seen" (Devarim 4:9). One might think that this applies even to one whose studies were too difficult for him, so the verse continues "and lest [the things you have seen] be removed from your heart, all the days of your life." So a person is not liable for his soul until he deliberately removes them from his heart.
In volume 2 of his Chesed LaAlafim, Rabbi Eliezer Papo ties these two propositions together via the Arizal. In Sha'ar HaMitzvot, at the end of parashat Vayelech, the Arizal is quoted as issuing a serious warning against eating the heart of any animal, domestic or wild, or of any bird. The Kabbalistic reason is that part of the ru'ach behemit ("animal spirit") of the animal or bird might enter the person who eats it and cause him to forget his Torah. The Chesed LaAlafim also refers to the other causes of forgetfulness listed in Horayot.
Eating an animal's heart is not forbidden by halachah and the Shulchan Aruch/Rema explain how it can be prepared for consumption. However, the aggadic passage in Horayot affirms a wider principle: if something is deleterious to one's memory, or to the learning process as a whole, one should avoid its regular consumption.
In modern terms, this suggests that anyone who seriously and sincerely wishes to preserve his or her memory and/or learn Torah should not indulge in mind-altering substance abuse. Taking drugs or consuming alcohol may offer perceptions and sensations that are not available to the conscious mind, but they come at a price -- and that price might well involve memory loss and cognitive impairment. And if a person takes mind-altering substances in the knowledge that his ability to remember things may be damaged, it is as though he is deliberately and willingly accepting the possible consequences of his actions.
The aggadah in Horayot is expressed in terms that will probably seem quaint and unfounded to many modern readers, but the mishnah in Avot has a more direct message for us today.
