mea_, iii. 124]

[Footnote 2: The truth is, that we see much less than is
commonly supposed. Not every impression is attended to that
is made on the retina, and unless we do attend we cannot,
properly speaking, be said to see. Walking across to college
one day, I was startled by seeing on the face of a clock in
my way that it was ten minutes to twelve, whereas I generally
passed that spot about twenty minutes to twelve. I hurried
on, fearing to be late, and on my arrival found myself in very
good time. On my way back, passing the clock again, I looked
up to see how much it was fast. It marked ten minutes to
eight. It had stopped at that time. When I passed before I
had really seen only the minute hand. The whole dial must have
been on my retina, but I had looked at or attended to only
what I was in doubt about, taking the hour for granted. I am
bound to add that my business friends hint that it is only
absorbed students that are capable of such mistakes, and that
alert men of business are more circumspect. That can only be
because they are more alive to the danger of error.]

CHAPTER III.

ASCERTAINMENT OF FACTS OF CAUSATION.

I.--_POST HOC ERGO PROPTER HOC_.

One of the chief contributions of the Old Logic to Inductive Method
was a name for a whole important class of misobservations. The fallacy
entitled _Post Hoc ergo Propter Hoc_--"After, therefore, Because
of"--consisted in alleging mere sequence as a proof of consequence or
causal sequence. The sophist appeals to experience, to observed facts:
the sequence which he alleges has been observed. But the appeal is
fallacious: the observation on which he relies amounts only to this,
that the one event has followed upon the other. This much must be
observable in all cases of causal sequence, but it is not enough for
proof. _Post hoc ergo propter hoc_ may be taken as a generic name for
imperfect proof of causation from observed facts of succession.

The standard example of the fallacy is the old Kentish peasant's
argument that Tenterden Steeple was the cause of Goodwin Sands. Sir
Thomas More (as Latimer tells the story in one of his Sermons to
ridicule incautious inference) had been sent down into Kent as a
commissioner to inquire into the cause of the silting up of Sandwich
Haven. Among those who came to his court was the oldest inhabitant,
and thinking that he from his great age must