Thomas Nagel's discussion of brain injuries, intelligent persons, and infants is found in his article "Death." In this article, Nagel discusses the different concepts of death, the impact of death, and different types of death.
On page 77 of Nagel's "Death," the following statement is made:
"Suppose an intelligent person receives a brain injury that reduces him to the mental condition of a contented infant, and that such desires as remain to him are satisfied by a custodian, so that he is free from care."
In this, Nagel means to illustrate the "fact" that the person with the brain injury does not suffer in the same way those around him or her suffer. Those around the person suffering from the brain injury are the only ones who realize how much change has happened. They are the ones responsible for the care and well-being of the injured person. Like an infant, the person with the brain injury is ignorant to the fact that he or she is different. Essentially, he or she is able to live "free from care" based upon the cliche "ignorance is bliss." The person has no clue as to how he or she was prior to the injury. Therefore, he or she has nothing to concern himself or herself with.
The likeness to an infant compounds when one considers the fact that the person with the brain injury must now be completely cared for by those around him or her. Essentially, some people with traumatic brain injuries cannot feed themselves or care for themselves at all. They are completely dependant on those around them in order to insure their survival.