Does a lack of devotion or absentmindedness during prayer invalidate it, and are only the prayers in which the worshiper was fully present counted for him?

"According to the majority of scholars, solemnity (khushu') in prayer is not a condition for its validity. Therefore, a lack of khushu' does not invalidate the prayer nor does it necessitate its repetition. However, it diminishes the reward of the prayer to the extent of the diminished khushu'. Errors in reciting the Quran do not invalidate the prayer unless they occur in Al-Fatihah and alter its meaning. If that is the case, then the prayer must be repeated if the error can be rectified; otherwise, the entire prayer must be redone. Abandoning prayer means not performing it at all, whereas performing it without khushu' fulfills the obligation, even if the reward is lessened."