Whose “normal mental and physical faculties” are we judged by, and what is “normal”?

According to the Texas DWI statute, “normal mental and physical faculties” refer to those of the particular person who has been arrested. The term does not refer to the normal faculties of the arresting officer, of the jurors, or of a fictitious average person. Indeed, the term “normal” actually refers to a range of measurement of the faculties of the actual person arrested in this case.