Thus, when G—— detailed to us his mode of searching the
      premises at the Hotel D——, I felt entire confidence in his having
      made a satisfactory investigation—so far as his labors extended.”

      “So far as his labors extended?” said I. “Yes,” said Dupin. “The measures adopted were not only the best
      of their kind, but carried out to absolute perfection. Had the
      letter been deposited within the range of their search, these
      fellows would, beyond a question, have found it.”

      I merely laughed—but he seemed quite serious in all that he said. “The measures, then,” he continued, “were good in their kind, and
      well executed; their defect lay in their being inapplicable to
      the case, and to the man. A certain set of highly ingenious
      resources are, with the Prefect, a sort of Procrustean bed, to
      which he forcibly adapts his designs. But he perpetually errs by
      being too deep or too shallow for the matter in hand; and many a
      schoolboy is a better reasoner than he. I knew one about eight
      years of age, whose success at guessing in the game of ‘even and
      odd’ attracted universal admiration. This game is simple, and is
      played with marbles. One player holds in his hand a number of
      these toys, and demands of another whether that number is even or
      odd. If the guess is right, the guesser wins one; if wrong, he
      loses one. The boy to whom I allude won all the marbles of the
      school. Of course he had some principle of guessing; and this lay
      in mere observation and admeasurement of the astuteness of his
      opponents. For example, an arrant simpleton is his opponent, and,
      holding up his closed hand, asks, ‘are they even or odd?’ Our
      schoolboy replies, ‘odd,’ and loses; but upon the second trial he
      wins, for he then says to himself, ‘the simpleton had them even
      upon the first trial, and his amount of cunning is just
      sufficient to make him have them odd upon the second; I will
      therefore guess odd;’—he guesses odd, and wins. Now, with a
      simpleton a degree above the first, he would have reasoned thus:
      ‘This fellow finds that in the first instance I guessed odd, and,
      in the second, he will propose to himself, upon the first
      impulse, a simple variation from even to odd, as did the first
      simpleton; but then a second thought will suggest that this is
      too simple a variation, and finally he will decide upon putting
      it even as before. I will therefore guess even;’—he guesses even,
      and wins. Now this mode of reasoning in the schoolboy, whom his
      fellows termed ‘lucky,’—what, in its last analysis, is it?”

      “It is merely,” I said, “an identification of the reasoner’s
      intellect with that of his opponent.”

      “It is,” said Dupin; “and, upon inquiring of the boy by what
      means he effected the thorough identification in which his
      success consisted, I received answer as follows: ‘When I wish to
      find out how wise, or how stupid, or how good, or how wicked is
      any one, or what are his thoughts at the moment, I fashion the
      expression of my face, as accurately as possible, in accordance
      with the expression of his, and then wait to see what thoughts or
      sentiments arise in my mind or heart, as if to match or
      correspond with the expression.’ This response of the schoolboy
      lies at the bottom of all the spurious profundity which has been
      attributed to Rochefoucault, to La Bougive, to Machiavelli, and
      to Campanella.”

      “And the identification,” I said, “of the reasoner’s intellect
      with that of his opponent, depends, if I understand you aright,
      upon the accuracy with which the opponent’s intellect is
      admeasured.”

      “For its practical value it depends upon this,” replied Dupin;
      “and the Prefect and his cohort fail so frequently, first, by
      default of this identification, and, secondly, by
      ill-admeasurement, or rather through non-admeasurement, of the
      intellect with which they are engaged. They consider only their
      own ideas of ingenuity; and, in searching for anything hidden,
      advert only to the modes in which they would have hidden it. They
      are right in this much—that their own ingenuity is a faithful
      representative of that of the mass; but when the cunning of the
      individual felon is diverse in character from their own, the
      felon foils them, of course. This always happens when it is above
      their own, and very usually when it is below. They have no
      variation of principle in their investigations; at best, when
      urged by some unusual emergency—by some extraordinary reward—they
      extend or exaggerate their old modes of practice, without
      touching their principles. What, for example, in this case of
      D——, has been done to vary the principle of action? What is all
      this boring, and probing, and sounding, and scrutinizing with the
      microscope and dividing the surface of the building into
      registered square inches—what is it all but an exaggeration of
      the application of the one principle or set of principles of
      search, which are based upon the one set of notions regarding
      human ingenuity, to which the Prefect, in the long routine of his
      duty, has been accustomed? Do you not see he has taken it for
      granted that all men proceed to conceal a letter,—not exactly in
      a gimlet hole bored in a chair-leg—but, at least, in some
      out-of-the-way hole or corner suggested by the same tenor of
      thought which would urge a man to secrete a letter in a
      gimlet-hole bored in a chair-leg? And do you not see also, that
      such recherchés nooks for concealment are adapted only for
      ordinary occasions, and would be adopted only by ordinary
      intellects; for, in all cases of concealment, a disposal of the
      article concealed—a disposal of it in this recherché manner,—is,
      in the very first instance, presumable and presumed; and thus its
      discovery depends, not at all upon the acumen, but altogether
      upon the mere care, patience, and determination of the seekers;
      and where the case is of importance—or, what amounts to the same
      thing in the political eyes, when the reward is of magnitude,—the
      qualities in question have never been known to fail. You will now
      understand what I meant in suggesting that, had the purloined
      letter been hidden any where within the limits of the Prefect’s
      examination—in other words, had the principle of its concealment
      been comprehended within the principles of the Prefect—its
      discovery would have been a matter altogether beyond question.