8127/frame/fm

particularly difficult in this respect and we must take great care to understand
their relevance to a crime clearly and completely.
In 
 
Morin
, fibers were collected from the body of a murdered girl and also
from the suspect’s car and house. Thousands of fibers were collected and a
few were selected that appeared to be shared between the body (a known
crime scene), the car (a suspected crime scene), and the house (a reference
environment for the suspect). Only after two trials and a judicial inquiry
was it clarified that the two families were neighbors, were occasionally in
each other’s houses and cars, and used the same laundromat. The judge
conducting the inquiry opined that the fibers should never have been col-
lected as their value from the start was worthless due to the shared envi-
ronments of the victim and suspect. That most of the matching fibers were
later suspected to have originated from contamination in the laboratory is
another matter.
We emphasize that 
 
searching
for evidence involves a different thought
process than 
 
interpreting
evidence. The former is deductive — we formulate
a theory based on previous experiences and information and use this working
hypothesis to help us know where to look. The philosophy and mind-set of
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Principles and Practice of Criminalistics
searching for evidence may upset some of you as it specifically transposes
the conditional (we condition on the hypothesis rather than the evidence)
and thus flies in the face of Bayesian thinking. But to interpret evidence, we
must have some, and successful searching requires presumptions, expecta-
tions, and preliminary hypotheses. In contrast, interpreting evidence uses
inductive reasoning — we synthesize all the evidence, including the results
of examinations and analyses — and develop a theory of the case. We suggest
that the Bayesian logic is most appropriate and helpful in the 
 
interpretation
of evidence. We discuss this topic in gory detail in Chapters 6 and 7.