The Necessity of
How can the attention of the heart and the expression of humility not be the spirit of the prayer and (how can) the perfection of prayer not depend upon them, while the musalli in his prayer and supplication whispers to his Lord?
- Mawla Narraqi
The Holy Qur'an says:
O you who believe
Do not near prayers in the state of intoxication
until you know what you say 
The Holy Prophet (s) is reported to have said:
Surely when the worshipper (musalli) prays,
He whispers unto his Blessed and Almighty Lord
Therefore he must know that with which he whispers. 
Prayer is a kind of worship taught to the human being by his Lord through Revelation to enable him attain His proximity. It consists of a set of actions and expressions, most of which clearly declare the immaculate nature of God.
A set of expressions, therefore, form an important part of the daily prayer. And clearly, the act of expression cannot really be known to be so if the heart is oblivious of what the tongue utters. Therefore, attention is a necessary concomitant of expression, and hence is also necessary in prayer.
Mawla al-Naraqi in his ethical opus, Jami' al-Sa'adat says:
How can the presence of the heart and the expression of humility not be the spirit of the prayer and (how can) the perfection of prayer not depend upon them, while the worshipper (musalli) in his prayer and supplication whispers to his Lord? There is no doubt that speech along with absent-mindedness is not the act of whispering. In addition, speech is an expression of what is in the heart or mind, and this is impossible save by means of the attention of the heart; for what (really) does the supplicant ask when he says: 'Guide us on the straight path' while the heart is inattentive? And there is no doubt also that the purpose behind Qur'an-recitation and invocations (adhkar) is the act of laudation, praise, the expression of one's humility and supplication while the addressee is the Almighty Allah. If the heart of the servant, therefore, was veiled from Him by the curtain of inattentiveness, and did not see or witness Him, but rather was inattentive of the addressee, and he moved his tongue out of (mere) habit, how remote then, is this, from the purpose behind prayer which was legislated for the refinement of the heart and renewal of one's remembrance of God... 
In addition, the following verse implies that the reason behind the prohibition of prayer in the state of intoxication is the worshipper's ignorance of what he utters:
O you who believe,
Do not near prayers in the state of intoxication
Until you know what you say 
Hence were he to pray in that state, he would utter statements the meaning of which he is oblivious.
An Etymological Discussion
Were we to reflect carefully on this verse, we would come to realize its universality: the word (sukara) is the plural of (sukran) and the word etymologically means:
"...interruption in a natural process, such that a course contrary to it follows ...and of its extensions is intoxication [ (sukr)) that comes about in the (natural) process of thinking and intellection..." 
The late mystic-scholar Sultan 'Ali, in his Qur'an commentary Bayan al- Sa'ada Fi Maqamat al-'Ibada says:
The word 'sukr' stems from the word 'sakr' meaning 'encumbrance;' and the state gotten by taking an intoxicant is known as 'sukr' because it encumbers and closes the channels of the intellect's influence on the other faculties of the human being and [hinders] the paths of their subordination to it. And intoxicants do not specifically relate to the well-known brandy [for example]; rather, all those things through which the state of intoxication is attained, whether by means of drinking, eating, smoking or other than that, are called the intoxicants of the nafs, no matter whether the common intoxication that comes about through the intake of beer and extracts gotten from other than grapes and the usage of narcotics and opium, is attained thereby or not; an example of the latter is when intoxication comes about through overwhelming greed, hope, love, desire, anger, jealousy, stinginess, sorrow, happiness, slumber or laziness, such that the requisites of these characteristics overpower the dictate of the intellect; rather (any) state gotten that hampers the execution of the intellect's command and control is sukrun nafs, regardless of its entity and cause ... 
`Abd al-Razzaq Qashani in his Qur'an commentary presents the following explanation for the verse under discussion:
(Do not near prayers): Do not near the station of presence and secret conversation with Allah when you are (intoxicated) by the sleep of negligence or the intoxicants of mundane desire and love of the world, (until you know what you are saying) in your whisperings and do not occupy your hearts with the occupations of the world and its insinuations .... 
Having known the roots of (sukr) we come to realize that the commonly understood intoxication is just one of its examples which the masses seem to be most familiar with. Other extensions (masadiq) can also be comprehended. Commentators of the Holy Qur'an mention a tradition of Imam al-Sadiq (`a) that implicitly endorses this contention. He (a) says about verse (4:43) that, of the examples of sukr is the 'sukr of sleep'. "This statement," says the Mawla Kashani in his 'al-Safi', "manifests universality."  He means that 'sukr' (that interruption that comes about in the natural process of thinking and intellection) has various extensions. In a tradition from Al-Kafi, Imam al-Baqir (`a) is reported to have said:
Neither stand for prayer in the state of restlessness, or slumber; nor while you are over satiated; for verily (all) those are of the characteristics of hypocrisy; and indeed Allah has forbidden you to stand in prayer while you are intoxicated; and by intoxication He means 'the intoxication of sleep'
Furthermore, the reason for the prohibition of prayer in the state of intoxication, as indicated in the verse under discussion, clearly reveals the necessity of attention of what one expresses in prayer.
Other verses of the Holy Qur'an also reveal, although implicitly, the necessity of attention in prayer. For example, verse 14 of chapter Taha clearly tells us that prayer is a means of remembering Allah:
Keep up prayer
For remembering Me. 
Can remembrance ever be possible when there is no attention and concentration in prayer? Obviously not.
Evidence From The Traditions
The following traditions too prove the necessity of the presence of heart in prayer:
1. The Ahlul bayt (a) are reported to have said:
You have no share from your prayer
Save that portion in which you maintained the attention of your heart. 
2. The Holy Prophet (s) is reported to have said:
Allah does not accept the prayer of that servant of his
Whose heart is not present along with his body. 
3. Imam al-Baqir (`a) is reported to have said:
When you stand up in prayer,
You should pay attention towards it,
For verily only that portion of it shall be taken into account
In which you paid attention. 
What The Muslim Jurists Hold
According to Muslim jurists (fuq'aha), however, the presence of heart is not obligatory in prayer. For not all men can achieve and maintain attention throughout their prayer. It would also require time before one has complete control over his imaginations in prayer. Hence, according to the Islamic jurisprudence the state of continual concentration is not a condition the absence of which invalidates prayer. Nevertheless it is considered as highly recommended, such that some jurists allow the prayer to be delayed within the framework of the prayer time for the sake of achieving the state of the heart's attention (iqbal).
Therefore although the prayer of one who does not observe attention is correct and sufficient in absolving him from his obligation, it would not transport him spiritually. The overt actions of prayer would only relieve him from disobedience. For him, however, to soar to the Divine and undergo a transformation would require attention and concentration.
 Holy Qur'an 4:43
 There is subtle point implied in this holy narration. The musalli 'whispers' and not merely engages in any kind of communication with His Rabb (Lover, Creator, Possessor, Sustainer and Trainer). The act of 'whispering' refers to a special kind of conversation, where the secretive feelings of the heart are expressed near the beloved, and a sense of amity is experienced. Who is this Omnipotent Beloved to Whom every element is dependent at every moment of its existence, and who knows all the misdeeds of His sinful slave, yet He permits him to whisper to Him several times a day?
 Ahmad bin Muhammad bin Hanbal, Musnad Ibn Hanbal, v.2, p.483, tr. 6135, Dar alFikr, Beirut, 2nd ed.
 Mawla Muhammad Mahdi Naraqi, Jami' al-Sa'adat, vo1. 3, p. 325.
 Holy Qur'an, 4:43.
 Mustafawi, al-Tahqiq, vo1.5, pp.160-161
 Mulla Sultan 'Ali, Bayan al-Sa'adah, v.2, pp. 21-22.
 Ibn `Arabi, Tafsir Ibn 'Arabi, vol. 1, p. 143
 Mawla Muhsin Fayd al Kashani, Tafsir al-Safi, v. 1, p.453
 Holy Qur'an, 20:14
Ibn Fahd Hilli, Uddat al-Da'i, p. 168
 Abu Jafar al-Barqi, al Mahasin, v.1, p. 406.
 Thiqat al-Islam
al-Kulayni, al-Kafi, v.3, p.299