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A.
Evidence and the Law
As discussed briefly in the Introduction to this book, forensic science has no
existence outside of the law. Forensic scientists are invited by the law to assist
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Principles and Practice of Criminalistics
in establishing the elements of a crime. Except for 
 
identification evidence
,
which we will meet again in Chapter 6 we do this only 
 
indirectly
by providing
 
circumstantial
evidence. To reiterate part of our discussion from Chapter 1,
circumstantial evidence is frequently confused with weak evidence. Attorneys,
in particular, like to call any evidence that is strong and convincing 
 
direct
evidence
. For example, a quote from a recent news article reads “‘There’s a
great deal of direct evidence,’ 
 
[the attorney] said, including DNA evidence and
a confession [the suspect] made to his father and brother
” (Lavie, 1999). While
the confession may or may not be true, it is still considered direct evidence.
And, however rare the DNA profile is, its presence still does not prove that
the suspect committed the murder, nor does it tell us when the material was
deposited; it simply indicates the presence of the suspect at some time at a
location connected with the crime. Depending on the circumstances of the
crime, this may be very convincing, but that does not make it direct evidence.
Science may establish a fact that allows for an 
 
inference
that the element
is true, but not the fact of the element. Briefly, an inference consists of two
parts, a fact and some assumptions. An inference is not a fact; it relies on
facts, but it must also rely on assumptions.
For instance, red wool fibers are found on a body dumped out in the woods.
A suspect is apprehended and a red wool sweater found in his closet. The
criminalist examines both items and reports to the detective that they are
microscopically indistinguishable. The detective infers that the suspect is
the perpetrator. Science provides the facts (indistinguishable red wool
fibers); the detective provides the many assumptions (the fibers would
continue to be indistinguishable by further testing, no one else has a red
wool sweater with the same characteristics, the suspect wore the sweater
found in his closet when he perpetrated the crime, the fibers were trans-
ferred during the commission of the crime, the fibers persisted after the
crime, etc., etc., etc.) required to make the (very tenuous) inference that the
fibers on the body lead to the suspect as the killer.
We discuss this important concept in more detail in Chapter 8, but for now,
we emphasize that science is the only player who can contribute facts about
physical evidence. Were it not for our ability to provide this service, we would
quickly be uninvited to the party.