I must take issue with Nat Hentoff's article ''New Guidelines for Ending Lives'' {op-ed, Oct. 3}. He claims that the providing of food and water through an artificial feeding tube is supplying two of life's necessities and therefore should never be denied. Assuming the moral high ground, he paints a picture of those who would deny the use of feeding tubes as going against ''millennia of human priorities.''

I have served as a full-time chaplain in a nursing home for more than four years. My deep feelings about this issue have grown out of my personal contact with the people who are the subjects of this academic debate. Hardly a day goes by that I do not hold the hand and look into the face of someone sustained by artificial feeding. Not a week goes by that I do not have a conversation with a family regarding life-prolonging measures. I approach this issue as a pastor because each day I must touch the lives of these people as a pastor.

To choose not to insert a feeding tube or to remove a tube from hopelessly ill patients is not to deny them food and water. Such basic sustenance should be offered to everyone. But when the disease process makes it impossible to take food by mouth, one can choose not to artificially replace the natural process. Fr. Kevin O'Rourke, of the Center for Health Care Ethics, St. Louis University, and former vice-president and director of Medical and Moral Affairs, Catholic Hospital Association, puts it this way: ''By definition, a patient in an irreversible coma cannot eat and swallow and thus will die of the pathology in a short time unless life-prolonging devices are utilized to circumvent the pathology. Withholding artificial hydration and nutrition from a patient in an irreversible coma does not introduce a new fatal pathology; rather, it allows an already existing fatal pathology to take its natural course.''

Where is there hope? Can the ''hopelessly ill'' hope that their lives will be sustained indefinitely? No. But we can have hope that we will not be forced to live beyond our natural days. We can have hope that our dying will not be prolonged. Modern medicine will add new ways to save lives, and for this we can be thankful. But those same devices can also prolong the dying process. Grant that we may know the difference between the two.