38 C.F.R. § 4.15
The ability to overcome the handicap of disability varies widely among individuals. The rating, however, is based primarily upon the average impairment in earning capacity, that is, upon the economic or industrial handicap which must be overcome and not from individual success in overcoming it. However, full consideration must be given to unusual physical or mental effects in individual cases, to peculiar effects of occupational activities, to defects in physical or mental endowment preventing the usual amount of success in overcoming the handicap of disability and to the effect of combinations of disability. Total disability will be considered to exist when there is present any impairment of mind or body which is sufficient to render it impossible for the average person to follow a substantially gainful occupation; Provided, That permanent total disability shall be taken to exist when the impairment is reasonably certain to continue throughout the life of the disabled person. The following will be considered to be permanent total disability: the permanent loss of the use of both hands, or of both feet, or of one hand and one foot, or of the sight of both eyes, or becoming permanently helpless or permanently bedridden. Other total disability ratings are scheduled in the various bodily systems of this schedule.
38 C.F.R. §4.15