There are two kinds of circumstantial evidence.
In one kind presumption of guilt depends on a series of links forming a
chain. In the other, the circumstances are woven together like the
strands of a rope. That is the ideal case of circumstantial evidence,
because the rope still holds when some of the strands are severed. The
case against Penreath struck me as resembling a chain, which is no
stronger than its weakest link. The strongest link in the chain of
circumstances against Penreath was the footprints leading to the pit.
They had undoubtedly been made by his boots, but circumstances can lie
as well as witnesses, and in both cases the most plausible sometimes
prove the greatest liars. Take away the clue of the footprints, and the
case against Penreath was snapped in the most vital link. The remaining
circumstances in the case against him, though suspicious enough, were
open to an alternative explanation. The footprints were the damning
fact--the link on which the remaining links of the chain were hung.

"But the elimination of the clue of the footprints did not make the
crime any easier of solution. From the moment I set foot in the room it
struck me as a deep and baffling mystery, looking at it from the point
of view of the police theory or from any other hypothesis. If Penreath
had indeed committed the murder, who was the second visitor to the room?
And if Penreath had not committed the murder, who had?

"That night, in my room, I sought to construct two alternative theories
of the murder. In the first place, I examined the case thoroughly from
the police point of view, with Penreath as the murderer. In view of what
has come to light since the trial, there is no need to take up time with
giving you my reasons for doubting whether Penreath had committed the
crime. I explained those reasons to Superintendent Galloway at the time,
pointing out, as he will doubtless remember, that the police theory
struck me as illogical in some aspects, and far from convincing as a
whole. There were too many elements of uncertainty in it, too much
guess-work, too much jumping at conclusions. Take one point alone, on
which I laid stress at the time. The police theory originally started
from the point of Penreath's peculiar behaviour at the Durrington hotel,
which, from their point of view, suggested homicidal mania. To my mind,
there was no evidence to prove this, although that theory was actually
put forth by the defence at Penreath