half-east from that
position, consequently you could see the door across the length of
the billiard-table, but you couldn't see the floor by the said
table. I found I was always forgetting to ask intruders to carry my
letters down-stairs for the mail, so I concluded to lay them on the
floor by the door; then the intruder would have to walk over them, &
that would indicate to him what they were there for. Did it? No,
it didn't. He was a machine, & had habits. Habits take precedence
of thought.

Now consider this: a stamped & addressed letter lying on the floor
--lying aggressively & conspicuously on the floor--is an unusual
spectacle; so unusual a spectacle that you would think an intruder
couldn't see it there without immediately divining that it was not
there by accident, but had been deliberately placed there & for a
definite purpose. Very well--it may surprise you to learn that that
most simple & most natural & obvious thought would never occur to
any intruder on this planet, whether he be fool, half-fool, or the
most brilliant of thinkers. For he is always an automatic machine &
has habits, & his habits will act before his thinking apparatus can
get a chance to exert its powers. My scheme failed because every
human being has the habit of picking up any apparently misplaced
thing & placing it where it won't be stepped on.

My first intruder was George. He went and came without saying
anything. Presently I found the letters neatly piled up on the
billiard-table. I was astonished. I put them on the floor again.
The next intruder piled them on the billiard-table without a word.
I was profoundly moved, profoundly interested. So I set the trap
again. Also again, & again, & yet again--all day long. I caught
every member of the family, & every servant; also I caught the three
finest intellects in the town. In every instance old, time-worn
automatic habit got in its work so promptly that the thinking
apparatus never got a chance.

I do not remember this particular discussion, but I do distinctly recall
being one of those whose intelligence was not sufficient to prevent my
picking up the letter he had thrown on the floor in front of his bed, and
being properly classified for doing it.

Clemens no longer kept note-books, as in an earlier time, but set down
innumerable memoranda-comments, stray reminde