applies we know for certain that a witness is telling the truth. In other words, if
the causal law applies for certain there is certain truth. However, it is unlikely in
practice that we know for certain that a witness is telling the truth.
Second, it helps us in the case where we do not know for certain that this
causal law applies. How Theorem 1 can help in this situation is covered by
Theorem 2, which we now consider.
Evidentiary Theorem 2
Evidentiary Theorem 2 is derived from Theorem 1. Theorem 1 proclaims that
a witness is telling the truth when the three conditions of truth are satisfied.
Since there is certain truth when the three conditions certainly apply, the
probability that they apply is also the probability that the witness is telling the
truth. This is Theorem 2. Theorem 2 says that the probability that a witness is
telling the truth is the probability that all three conditions of truth are satisfied.
Theorem 2 yields a highly useful consequence. It means that a court can
assess the probability that evidence is true by considering the factors that
affect how fully each condition is satisfied. For example, an independent
witness who chances to come on the scene of a crime being committed has, in
general terms, less motive to be untruthful than the husband of the criminal
who may be strongly tempted to lie to protect the woman he loves. (This
example, by the way, shows the two sides of the task. On the one hand, it is
not scientific. On the other hand, there is common wisdom and common
sense, which is a good guide, even if it is not infallible or capable of
quantitative measurement.)
Condition 1: Fully Observing Facts
Condition 1 for a witness to tell the truth is that the witness has fully observed
the facts. Faculties to observe are sight, hearing, touch, taste and smell.
Without claiming to be comprehensive, the obvious requirements for a witness
to fully observe the facts are the following:
(1)
The observational faculties of the witness must function properly.
(2)
The witness was present at the events.
(3)
The witness was in a position
to observe these events fully and
properly. So for each witness there will be variations on probability according
to the quality of the observation (for example, how long, how good, how far
away and so on).
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(4)
The witness in fact observed everything.
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Some of the issues involved here are discussed in Re (1984).
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In this regard it will be obvious that evidence by a witness based on
another person’s observation is generally weak. Witness A says that Fact X is true
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