You don't see angels nowadays the way people used to in biblical times. The job of bringing messages from outside our tiny, human corner of reality is more likely to be entrusted to folks in spaceships who may look a bit odd but are clearly some sort of flesh and blood. Mortimer Adler, who is probably the best-known living disciple of Aristotle and St. Thomas Aquinas, devotes some of his space to these extraterrestrial forms of nonhuman intelligence, but you can hear a kind of condescension in his tone of voice whenever the subject comes up. He does not exactly say that we cannot believe in little green men, but to a man whose eyes are fixed on heaven, anyone who travels by spaceship just doesn't seem very important.

The trouble with being a human (or a Plutonian) rather than an angel is that we are stuck with all this physical apparatus--arms and eyes, hearts and brains and stomachs and reproductive equipment--whose demands often interfere with the processes of pure perception, meditation and decision. Adler puts it succinctly in a sentence that illustrates well his rather dry and seemingly all-knowing, all-pondering style: "Unlike minds without bodies, infinite or finite, minds dependent on bodies cannot understand, know or think without the concurrence of bodily functions."

Never mind that the end of this sentence leaves us precisely where we were at the beginning (like the definition: "Community singing is singing by the community"). This is a fairly common process in philosophical systems that rely heavily on pure deduction--which sometimes resemble an attempt to build a house from the roof down. Adler's is such a system. He spends most of his time reading what others have said on the subject, turning it over in his mind and trying to add his own little refinements here and there without ever really looking around to make his own fresh observations based on fresh empirical data. It is the system of thought under which generations of otherwise respectable minds assumed that the Earth was not only flat but the center of the universe, the focal point around which the sun revolved.

In fairness to Mortimer Adler, it must be admitted that empirical methods of study do not get us very far when we approach the subject of angels. You cannot dissect an angel cadaver, conduct a survey of opinion among angels, or interview one. What you have to do in all practicality is examine what has been written by those who may have known more or thought more about the subject. Outside of examining the contents of your own mind (and a variety of Holy Scriptures, if you want to venture beyond pure philosophy into theology), there is simply no other source of raw material. And the material available in books, holy or otherwise, is hardly fresh. "The modern thinker who, in my judgment, has made the most significant philosophical contribution to the subject is John Locke, in his 'Essay Concerning Human Understanding,' " says Adler. Locke died in 1704.

"Modern" is, of course, a relative term, particularly in the timeless world of the scholastic philosopher. There are trivial details here and there that indicate that Mortimer Adler is a man of the 20th century, but the bulk of his book could have been written (has, in fact, been written by various authors) at any time after the work of its principal source and inspiration, Thomas Aquinas--that is, any time during the last seven centuries. It is mildly amazing to see it written (and issued by a serious, reputable publisher) in the 1980s.

In his preface, Adler explains how it happened--beginning back in 1945 when he was working on the "Syntopicon," a systematic index of the Great Ideas contained in the "Great Books of the Western World." He narrowed the field down to exactly 102 Great Ideas and began to write them up, beginning (in alphabetical order) with "ANGEL." The guiding spirits at the Encyclopaedia Britannica, sponsor of the project, were "flabbergasted at my choice of ANGEL as one of the great ideas," Adler reports, particularly Sen. William Benton, who was then the publisher. "He thought it did not belong in that company at all. What made matters worse was the prominence given it by putting it first." Since then, he has lectured on angels before audiences of all kinds and found that "the subject had the same fascination for others that it did for me, a fascination that was in no way affected by the heterodox beliefs of the persons listening or diminished by the absence or denial of any religious belief."

Translated into contemporary jargon, this means that there is a market for such a book--and the existence of a market (which might be considered for inclusion as Great Idea No. 103) has been considered sufficient justification for publishing a book. The potential importance of the subject is also a factor: If, in fact, there are or may be such things as angels and if we can learn anything useful about them, the subject should be one of the hottest in publishing.

What Adler demonstrates, however, is somewhat less compelling. He argues quite convincingly that we cannot prove that angels do not exist (it is almost impossible, of course, to prove the nonexistence of anything the idea of which does not contain internal contradictions), and he discusses in a fashion both orderly and dryly eloquent the various convolutions available to the human mind when it chooses to think about angels. It is interesting enough, in a crossword-puzzle sort of way, for those who are attracted to the subject--but ultimately the perceptible additions to the sum of human knowledge seem hardly worth the effort involved.

I suspect that if Adler had spent all those years lecturing on UFOs rather than angels, he would have encountered an equally eager audience response--just as books on UFOs are a much hotter market item than books on angels. Perhaps this is not as it should be, but at least the UFO books usually claim to offer eyewitness reports.