THE ''quality of life'' began its career as a rather innocuous phrase. Typically it served as a catchall piety a politician might deliver in the last paragraph of a speech, performing rousing lip service to the virtues of clean air, pure water, decent public schools - almost anything that could be thought of as ''life enhancing,'' to ring in another cliche.
More recently ''quality of life'' has taken on a special meaning. Without becoming more precise, the phrase now serves as a sort of code to signal that a judgment is about to be passed on when human existence is worth continuing - and when it is not.
Thus ''quality of life'' is the standard appealed to in the ''Baby Jane Doe'' case when partisans try to make a distinction between a genetic ''monster'' and a ''human being,'' to borrow their somewhat loaded terms.
''Quality of life'' is also the phrase that hovers over the now-famous statement of the governor of Colorado, Richard Lamm: ''Like leaves which fall off a tree forming the humus in which other plants can grow, we've got a duty to die and get out of the way . . . so that our kids can build a reasonable life.''
The argument runs: If an infant can be pronounced to be a ''monster,'' if an elderly person can be pronounced to be a ''vegetable,'' then a minimal ''quality of life'' can be declared to be lacking and, out of a higher compassion for all concerned as well as a consideration of the economics, the life that has been judged to be less than life ought to be ended.
Those making this argument believe they are being more rather than less humane. Dr. Daniel D. Federman, a professor at the Harvard Medical School, speaking for other physicians, has suggested ''we have a hard time letting go.'' Maintaining that ''the senseless perpetuation of the status quo'' is ''decision by default'' when ''quality of life'' is absent, he and a growing number of professionals - lawyers as well as doctors - want ''guidelines'' set up.
But what are these ''guidelines'' to be? The following phrases are used again and again to describe the cutoff point - the point at which treatment should cease for a ''Baby Doe,'' or the plug should be pulled on one of Governor Lamm's elderly. ''Hopelessly ill.'' ''Irreversible condition.'' ''Terminal situation.'' ''Needless suffering.'' These descriptions are as emotional as they are likely, in the end, to be subjective and inexact.
By taking extreme cases, those who support a policy of ''letting go'' make the judgment of ''quality of life'' seem a simple professional matter, scarcely more difficult than pronouncing a patient legally dead. It is assumed that most right-thinking people can agree when an end to human existence is ''appropriate.'' Few problems are anticipated as to ''adequate safeguards'' - a ''living will'' expressing in advance the preferences of the patient, or the ''informed consent'' of the nearest of kin.
But consider a few of the proposals already made for deciding when ''quality of life'' has sunk below an arbitrary minimum and care should be withheld. Not long ago a professor of ethics devised what he called ''Indicators of Humanhood.'' If, for example, one's IQ dropped below 40, one would cease to be regarded as fully human and become subject to the less inhibited treatment due a ''vegetable.'' IQ, as we have lately learned, is a questionable measure in itself. But who would want to be the bureaucrat to say, ''41 - you live . . . ah , but you, there, 39, your 'quality of life' is off the scale, and the dignity of everybody's 'humanhood' requires our 'letting go' of you.''
It is worth reminding ourselves that ''quality of life'' is a cultural judgment. People as enlightened as the ancient Greeks abandoned babies to die because of minor deformities. In some cultures being a baby girl was regarded as deformity enough.
Nobody should pretend that in 1984 things are wholly different - that now there is a science to judging the boundary lines of the ''quality of life'' just because terms like ''bioethics'' are becoming popular.
Writing in the Village Voice, Nat Hentoff has accumulated case histories of children who would have been ''let go'' as Baby Does if they had been subjected to the usual ''quality of life'' formulas. He quotes one adopting parent: ''The creative unfolding of their human spirit allows them to belong, to be fully alive. . . . Each child can bring joy and be joyful.''
The debate, at the moment, is full of haste and shrill generalization. Name-callers have stooped to characterizing opponents as ''death doctors,'' on the one hand, and advocates of ''New Right yahooism,'' on the other. Both sides claim to stand for civil rights.
We all need to calm down - and slow down. Everybody's instinct, as usual, is to resolve the issue as quickly and neatly as possible - to pass a law. But there are issues where that temptation ought to be resisted and the wrestling be allowed to go on until further wisdom is revealed. This may be one of those issues.
Meanwhile, out of respect for those personally involved, whose decision is not a debater's abstraction, we bystanders ought to consider, and reconsider, the consequences of giving any individuals - including Our Side - the power to decide for other individuals whether their ''quality of life'' is tolerable. For who can doubt that any precedents set will be more readily expanded upon than reversed?