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b.
Physical Evidence and Circumstantial Evidence
Before we proceed further, it is worth reviewing the concepts of 
 
circumstan-
tial evidence
and 
 
physical evidence
. Most physical evidence is circumstantial
evidence — that is, its involvement in the scenario requires some further
inference or assumption. Exceptions to this generality include substances that
by their very presence are illegal, such as illicit drugs. Circumstantial evidence
need not be physical, although most of it is. The term circumstantial evidence
carries with it the connotation of untrustworthiness, while eyewitness evi-
dence tends to be considered conclusive. Nothing could be farther from the
truth. In fact, numerous studies have shown eyewitness evidence to be
remarkably unreliable (Loftus, 1996).
Consider the following scenario. You are camping in a cabin deep in the
woods in Alaska. It is the depth of winter and the snow pack is many feet.
You hear a sound in the middle of the night and go to the window to see
what it is. The moon is new, and it is practically pitch dark. You see a quickly
moving shadow that seems to blend in with the snow. Is it a polar bear? Is
it a potential burglar dressed in white? You really can’t be sure. In the morning
you venture outside to behold — very clear snowshoe tracks. There is no
question in your mind that the previous night’s visitor was a human, not a
bear. You then also notice that the lock to your toolshed is hanging open,
and a reconnaissance of the interior reveals that some of your tools are
missing. Which evidence was more convincing — your midnight glimpse
(eyewitness evidence) or the prints in the snow (circumstantial evidence)
combined with the burglary? More important, which evidence will be more
convincing to the local police — your description of a dark blur or casts of
the tracks in the snow. Even if you had had a really good look at the burglar,
it’s still your word against his; the physical evidence of the tracks is inarguable.
(The issue of the significance of a match to the suspect’s snowshoes is another
issue). So much for the assertion that “the evidence is only circumstantial.”
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Principles and Practice of Criminalistics