8. Davening from memory:

A. The Halachic issue:^[1]

It is forbidden to recite by heart parts of the written Torah which most people do not know by heart. However, the custom is to allow reciting by heart the Song of the Sea (Shiras Hayam), Songs which contain verses, the Tamid paragraph, the Shema paragraph, and psalm, as these paragraphs are known by most people by heart. Furthermore, some Poskim rule that it is only forbidden to recite by heart parts of the written Torah when ones recitation assists another person to fulfill his obligation. Accordingly, one should not protest those people who say to themselves verses by heart even if they are not well known.

May a blind person pray, and learn the written Torah by heart? According to all opinions, a blind person may recite any part of Torah by heart.

Tehillim:^[2] According to all, one may say Tehillim by heart.

B. The recommendation:^[3]

Some are accustomed to specifically pray from within the Siddur for greater concentration, and some are accustomed to specifically Daven by heart with their eyes closed for greater concentration. Practically, whether one should pray by heart with his eyes closed, or from reading from the Siddur is dependent on whatever will help one have greater concentration which may fluctuate from day-to-day. Nonetheless, even one who chooses to pray with his eyes closed should at least have the Siddur open in front of him.

[1] See Admur 49; Ketzos Hashulchan 14:2; Igros Kodesh 24:24; Shulchan Menachem 1:41

[2] Beir Heiytiv 49 in name of Chavos Yair; Ketzos Hashulchan 14:2; Hayom Yom

[3] Admur 49; Ketzos Hashulchan 14:2