True that the presidential prelims are full of evasions, demagogy and blather. But you do learn a little about the character of our candidates, so that it can be said that all is not lost in democratic epistemology.

Consider, for instance, Gary Hart's behavior last weekend when the Democratic candidates met to debate under the baton of John Chancellor. Chancellor is learned, cosmopolitan, fluent and very bright, and in his introduction to Gary Hart he said jocularly that Hart's reentry into the presidential race had raised the question, ''Will you love me in December as you did in May?'' That was a very nice grace note, and whaddayaknow, Hart reacted as though his virility had been questioned, which is the last question anyone is asking about Gary Hart. He ran down the litany of Hart Enterprises Inc. (world peace, world prosperity, interracial harmony and growth) and then said, ''I should say, Mr. Chancellor, that's more important in people's minds than the questionable taste of your introduction.''

You walk away from that one wondering whether Hart has any sense of proportion. If he can be upset by Chancellor's quoting a line from an old song, one wonders how a President Hart would react to the repetition of far more insistent lines by percussive Soviet instruments.

And then Bruce Babbitt of Arizona had a nice little temper flare when his credentials as a Democrat were challenged. What happened was that the subject was raised: How would the Democrats raise money to reduce the deficit and to give everybody a free telephone? And Babbitt said that the time may have come for a national consumption tax.

Albert Gore pounced: A consumption tax, he said, is a Republican idea. The first consumption tax I know of, and my memory on the matter is finite, was imposed in Greece at about the time Socrates was drinking hemlock. But Babbitt chose to react as though his legitimacy was being questioned. He responded that ''nobody questioned my credentials as a Democrat'' when he was marching for civil rights in the South in the 1960s, which is a technique known to the rhetoricians as ignoratio elenchi: you accuse someone of being a wino and he responds that he never smoked a cigarette in his entire life.

During the weekend the candidates met a cross section of American voters by radio and television, submitting to questions from five families. The launch wobbled just a little when it transpired that the first woman to ask a question didn't know what George Bush ''does for a living.'' Bush might have answered the question wryly, as Theodore Roosevelt once did when asked what on earth a vice president did (''Nothing''); but one does not safely venture into Stevensonian wisecrackery in primary elections, and no doubt Bush patiently explained that there was an office called ''vice president,'' etc., etc.

Though a little wit is not entirely proscribed, it was nicely used by Gore when he had to live through yet one more recitation about how Michael Dukakis had saved the economy of the Commonwealth of Massachusetts, which he and his supporters like to call ''the Dukakis miracle.'' This designation was once too many for Al Gore, who said, ''I went to divinity school, and they have a tougher definition of the word 'miracle.'''

The most serious exchange that sifted through was the question asked of Sen. Paul Simon. The questioner wanted to know if Simon would order an immediate retaliatory strike in the event Soviet missiles were detected approaching the United States.

Simon did his best to avoid that one and rattled on about de-escalation and disarmament and avoiding an arms race. But the questioner persisted: ''You really didn't answer the question,'' he said.

''I understand,'' Simon replied, ''and my answer is that no responsible candidate can answer 'What if?' to military questions.''

Simon has it quite wrong. No responsible candidate can answer that he would decline to retaliate against Soviet nuclear aggression. Because any ambiguity has the effect of unilateral nuclear disarmament, which leaves us wondering whether we can love Simon as much in January as we did in May.