Leverage frowned thoughtfully.

“Listens good,” he volunteered at length.  “Another thing—­Barker admits he was shooting pool in Kelly’s place last night around midnight; and Kelly’s place is only half a block from the Union Station.  That sounds significant!”

“It does; and then again it may mean nothing.  What I am striving for is to make William Barker feel that he is safe.  The safer he feels, the more readily he will talk.  No matter how many lies he tells, everything that he says is of value.  He didn’t know, of course, that we already had a perfect alibi for Miss Gresham; but even if we hadn’t, his assumed belief that she committed the crime would have assured me that she did not.  No-o, I think we’d better not arrest the man unless he forces our hand—­tries to jump town, or something like that.  Better let him remain at large and talk frequently.  If he has anything to betray, there’s more chance that he’ll do it that way.  Don’t you think I’m right?”

“I wouldn’t admit it if I didn’t, Carroll.  I’ve seen you in action too often to believe you’re ever wrong.”

Carroll flushed boyishly.

“Don’t be absurd, Leverage!  I’m often wrong—­very wrong.  And don’t think that I’m a transcendent detective; they don’t really exist, you know.  I’m merely trying to be human, to learn the nature of the people with whom I’m dealing.  I try to learn ’em as well as they know themselves—­maybe a little better; and then I try to separate the wheat of vital facts from the chaff of the inconsequential.”

“Just the same,” insisted Leverage loyally, “you always get ’em!”

“And when I do, it is because I have used nothing more than plain common sense.  Don’t think that I attach no importance to physical clues.  They’re immensely valuable; but the one weakness in a criminal is his lack of common sense.  His perspective is awry, his sense of values distorted.  Usually he bothers his head about a myriad minor details, and pays but scant attention to the genuinely important things.  It is upon that weakness that I am banking—­particularly so in the case of Barker.”

“I insist that you’re a wonder, Carroll!”

“And I insist that you’re foolishly complimentary.  Did you ever stop to realize, Eric, that when a crime is committed the advantage lies entirely with the detective?  The detective can make a thousand mistakes during the course of his investigations and still trap his man; but the criminal cannot make one single error—­not one!”

“Maybe so, David; but it takes a good man to recognize that one, and to know what to do with it.”