"'And when did this happen?' said I. 'For, as a matter of fact, I have heard nothing of the disappearance of an express; and it ought to be generally known.'
"'It must be known by now,' he, said in a very melancholy tone. 'It only happened a few hours before our fall into the Catacombs.'
"I examined the paper once more for quite five minutes. I reflected deeply, asked for certain complementary details, and then burst out laughing: though in truth it was not a laughable matter, for the catastrophe was truly appalling. What made me laugh was the seeming difficulty of the problem and the delight of having solved it in five minutes.
"'You believe yourself a rational human being,' I cried, 'because you have Reason! But you're exactly like ninety-nine people out of a hundred, you don't know how to make use of it. You talk of Reason; but what use is Reason in a brain which does not know by which end to take hold of it? It's a wonderful instrument in the hands of a doll! Don't turn away your head in that sulky way, M. Longuet. I tell you: you don't know by which end to take hold of your Reason! Come, M. Longuet: let us reason with this paper in our hands.'
"He tried, the duffer! He said: 'There were five men at A, and five men at B. The five men at B saw the train pass; the five men at A did not see it. I—I was at K; and I am sure that it did not pass at K... consequently...'
"'Consequently?... Consequently, there's no longer any express? Consequently your express has vanished—melted—flown away? Hey, presto: vanish express! You think perhaps that the express is in the English Channel! You see clearly, M. Longuet, that if you have Reason, you don't know how to use it. Allow me to tell you that you took hold of your Reason by the wrong end! The wrong end is that which begins by saying, 'We did not see the express,' and which ends by saying, 'Then there is no longer any express!' But I am going to show you how to take hold of your Reason by the right end. It is this: the truth is that the express exists, and that it exists between the points B, where it was seen to pass, A, where it was not seen to pass, and I, where it could not pass. Since we are in a plain, your express is between A, B and I. That is certain...'
"'But!'
"'Hush! Be quiet! And since we are in a plain, and in that plain there is an immense mass of loose sand, the only place in which the train could have disappeared is in that mass of sand: that is the eternal truth!'...
"'I swear it didn't! I was at K waiting for the express; and I did not quit the line H I.'
"'By the immortal Masterpieces of the Italian Renaissance I command you not to let go of the right end of your Reason which I have put into your hand. We are discussing at this moment what is; we are not yet at the how. It is owing to the fact that you began with the how that you have not been able to reach what is. The express is in I, since it cannot be anywhere else. I am sure that the five men could not have seen it pass B, as they assert, unless it had passed. I am as certain that five men could not have been unable to see it at A if it had passed A; and since the line A B was examined and found not to contain the express, it must be that it turned off up the line H I. There we are, then, with the train on the line H I.'