The definition of a talmid chacham

Prompted by a post on Shimush Chachamim... (And soon we'll get up to it in daf yomi.)

The relevant gemara (Brachos 4b-5a):

א"ר יהושע בן לוי אע"פ שקרא אדם ק"ש בביהכ"נ מצוה לקרותו על מטתו

אמר רבי יוסי מאי קרא (תהלים ד, ה) רגזו ואל תחטאו אמרו בלבבכם על משכבכם ודומו סלה

אמר רב נחמן אם תלמיד חכם הוא אין צריך 

אמר אביי אף תלמיד חכם מיבעי ליה למימר חד פסוקא דרחמי כגון (תהלים לא, ו) בידך אפקיד רוחי פדיתה אותי ה' אל אמת:

Or, in English:

R. Joshua b. Levi says: Though a man has recited the Shema' in the synagogue, it is a religious act to recite it again upon his bed. R. Assi says: Which verse [may be cited in support]? Tremble and sin not; commune with your own heart upon your bed, and be still, Selah.³⁴  R. Nahman, however, says: If he is a scholar, then it is not necessary. Abaye says: Even a scholar should recite one verse of supplication, as for instance: Into Thy hand I commit my spirit. Thou hast redeemed me, O Lord, Thou God of truth.¹

What does it mean "if he is a scholar"? Rashi writes:

ואם תלמיד חכם הוא - שרגיל במשנתו לחזור על גרסתו תמיד דיו בכך

Which prompts Shimush Chachamim's comment:

The p'shat in the gemorah is that since he is a Talmud Chacham and has the 'shmirah' of his Torah he doesn't need to lein Krias Shema before going to bed. Although we see in this how Rashi describes a Talmud Chacham....שרגיל במשנתו לחזור על גרסתו תמיד

I'm not so sure I agree with that peshat in the gemara, that this is about shmira, rather than avoidance of sin. I think that just as there are rationalists and non-rationalists today, there were rationalist and non-rationalist Amoraim. Or rather, sometimes they put forth rationalist reasons and sometimes 'non-rationalist' reasons.

In terms of the purpose of krias shema al hamita, the Amoraim were divided. The "rationalists" took it as a way of regulating one's emotions and thoughts before otherwise uncontrolled sleep, in order to avoid seminal emissions. The "mystics" took it as a guard against mazikin.

An example of a mystic is Rabbi Yitzchak. In Berachos 5a:

א"ר יצחק כל הקורא ק"ש על מטתו כאלו אוחז חרב של שתי פיות בידו שנאמר (תהלים קמט, ו) רוממות אל בגרונם וחרב פיפיות בידם מאי משמע אמר מר זוטרא ואיתימא רב אשי מרישא דענינא דכתיב (תהלים קמט, ה) יעלזו חסידים בכבוד ירננו על משכבותם וכתיב בתריה רוממות אל בגרונם וחרב פיפיות בידם. ואמר רבי יצחק כל הקורא קריאת שמע על מטתו מזיקין בדילין הימנו שנאמר (איוב ה, ז) ובני רשף יגביהו עוף ואין עוף אלא תורה שנאמר (משלי כג, ה) התעיף עיניך בו ואיננו ואין רשף אלא מזיקין שנאמר (דברים לב, כד) מזי רעב ולחומי רשף וקטב מרירי.

R. Isaac says: If one recites the Shema' upon his bed, it is as though he held a two-edged sword in his hand.⁷  For it is said: Let the high praises of God be in their mouth, and a two-edged sword in their hand.⁸  How does it indicate this? — Mar Zutra, (some say, R. Ashi) says: [The lesson is] from the preceding verse. For it is written: Let the saints exult in glory, let them sing for joy upon their beds,⁹  and then it is written: Let the high praises of God be in their mouth, and a two-edged sword in their hand. R. Isaac says further: If] one recites the Shema' upon his bed, the demons keep away from him. For it is said: And the sons of reshef¹⁰  fly ['uf] upward.¹¹  The word 'uf refers only to the Torah, as it is written: Wilt thou cause thine eyes to close [hata'if]¹²  upon it? It is gone.¹³  And 'reshef' refers only to the demons, as it is said: The wasting of hunger, and the devouring of the reshef [fiery bolt] and bitter destruction.¹⁴

Thus, the fear is mazikin, demons.

Examples of 'rationalists' include Rabbi Assi. Thus, as above:

אמר רבי יוסי מאי קרא (תהלים ד, ה) רגזו ואל תחטאו אמרו בלבבכם על משכבכם ודומו סלה

R. Assi says: Which verse [may be cited in support]? Tremble and sin not; commune with your own heart upon your bed, and be still, Selah.

From the beginning of the pasuk, we see Rabbi Assi's intent. Tremble and sin not. By directing one's heart to Hashem and thus trembling, and having this meditation in bed before sleep, then he will be still.

According to Rav Nachman, a rationalist, a Talmid Chacham does not to recite the Shema on his bed, because his mind is already focused on Hashem. He does not need the reminder. Thus, says Abaye:

אמר אביי אף תלמיד חכם מיבעי ליה למימר חד פסוקא דרחמי כגון (תהלים לא, ו) בידך אפקיד רוחי פדיתה אותי ה' אל אמת

Abaye says: Even a scholar should recite one verse of supplication, as for instance: Into Thy hand I commit my spirit. Thou hast redeemed me, O Lord, Thou God of truth.¹

What is this supplication, and committing of the spirit? We could read it as committing of the spirit to Hashem's protection, to save him from mazikin. Or, as I would understand it in context, this sort of supplication acts to establish a connection to Hashem and prevent sin.

(Alternatively, perhaps Abaye is a non-rationalist. We see elsewhere, Chullin 105b, that he gives rationalist explanations for certain religious acts, but then, based on his rebbe's say-so (either Rav Yosef or Rabba), concedes that it is due to more mystical reasons. For example:
ואמר אביי מריש הוה אמינא האי דלא אכלי ירקא מכישא דאסר גינאה משום דמיחזי כרעבתנותא אמר לי מר משום דקשי לכשפים.

Maybe Abaye is playing the non-rationalist here and saying that even though the rationalist concern does not apply, the non-rationalist one still does, and so one should make a short supplication for protection.
)

We can see the reason of avoiding sin, as the gemara continues:

R. Levi b. Hama says in the name of R. Simeon b. Lakish: A man should always incite the good impulse [in his soul]²  to fight against the evil impulse. For it is written: Tremble and sin not.³  If he subdues it, well and good. If not, let him study the Torah. For it is written: 'Commune with your own heart'.⁴  If he subdues it, well and good. If not, let him recite the Shema'. For it is written: 'Upon your bed'. If he subdues it, well and good. If not, let him remind himself of the day of death. For it is written: 'And be still, Selah'.

Same pasuk, yet about fighting the yetzer hara.

(Admittedly, there is an idea of Shema [and based on the derivation, any words of Torah], providing a protection from demons.)

Still, I agree that there is Rashi's definition of a Talmid Chacham here. Perhaps, or perhaps not, we will be able to extrapolate from it to talmidei chachamim all over Sha"s. Rashi said:

ואם תלמיד חכם הוא - שרגיל במשנתו לחזור על גרסתו תמיד דיו בכך

"And if he is a talmid chacham [he need not recite Shema on his bed]: For he is accustomed in his learning, to review his girsa always, it is sufficient in this."

In other words, based on context, Rashi is not merely giving a definition but explaining why. Since a Talmid Chacham is ragil bemishnato, we may assume that he is already thinking about his learning. And this would have the effect of not needing his mind diverted away from sin, and towards fear of sin. (Or from a non-rationalist perspective, his learning will protect him from demons and suffering in general.) But maybe that is only in this narrow context in the gemara, that we can surmise that this is what is meant. But other times talmidei chachamim might be used more loosely.