"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 retarding human ingenuity, to which the Prefect, in the long routine of his duty, has been accustomed? Do you not see he had 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 _recherché_ 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, 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 anywhere 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. This functionary, however, has been thoroughly mystified;