Moses Maimonides, The Guide for the Perplexed, New York 1956 p. 386-387 – Praying
We must bear in mind that all such religious acts as reading the Law, praying, and the performance of other precepts, serve exclusively as the means of causing us to occupy and fill our mind with the precepts of God, and free it from worldly business; for we are thus, as it were, in communication with God, and undisturbed by any other thing. If we, however, pray with the motion of our lip’, and our face toward the wan, but at the same time think of our business; if we read the Law with our tongue, whilst our heart is occupied with the building of our house, and we do not think of what we are reading; if we perform the commandments only with our limbs, we are like those who are engaged in digging in the ground, or hewing wood in the forest, without reflecting on the nature of those acts, or by whom they are commanded, or what is their object. We must not imagine that [in this way] we attain the highest perfection; on the contrary, we are then like those in reference to whom Scripture says, ” Thou art near in their mouth, and far from their reins ” (Jer. xii. 2).
I will now commence to show you the war how to educate and train yourselves in order to attain that great perfection.
The first thing you must do is this : Turn your thoughts away from everything while you read Shema’ or during the Tefilla, and do not content yourself with being devout when you read the first verse of Shema, or the first paragraph of the prayer. When you have successfully practised this for many years, try in reading the Law or listening to it, to have all your heart and all your thought occupied with understanding what you read or hear.
After some time when you have mastered this, accustom yourself to have your mind free from all other thoughts when you read any portion of the other books of the prophets, or when you say any blessing ; and to have your attention directed exclusively to the perception and the understanding of what you utter. When you have succeeded in properly performing these acts of divine service, and you have your thought, during their performance, entirely abstracted from worldly affairs, take then care that your thought be not disturbed by thinking of your wants or of superfluous things. In short, think of worldly matters when you eat, drink, bathe, talk with your wife and little children, or when you converse with other people. These times, which are frequent and long, I think, must suffice to you for reflecting on everything that is necessary as regards business, household, and health. But when you are engaged in the performance of religious duties, have your mind exclusively directed to what you are doing.
When you are alone by yourself, when you are awake on your couch, be careful to meditate in such precious moments on nothing but the intellectual worship of God, viz., to approach Him and to minister before Him in the true manner which I have described to you-not in hollow emotions. This I consider as the highest perfection wise men can attain by the above training.
When we have acquired a true knowledge of God, and rejoice in that knowledge in such a manner, that whilst speaking with others, or at tending to our bodily wants, our mind is all that time with God; when we are with our heart constantly near God, even whilst our body is in the society of men; when we are in that state which the Song on the relation between God and man poetically describes in the following words: ” I sleep, but my heart waketh; it is the voice of my beloved that knocketh ” (Song v. 2) then we have attained not only the height of ordinary prophets, but of Moses, our Teacher, of whom Scripture relates : ” And Moses alone shall come near before the Lord ” (ibid. xxxiv. 28); ” But as for thee, stand thou here by me” (Deut. v. 28).