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Only an expert witness can testify to something other that what he saw or heard. An
expert can provide opinions on technical matters in areas such as medicine, engineering,
etc. This is permitted because it is not reasonable to expect a judge or jury to be able
to form their own opinions or conclusions on technical matters.
All evidence, other than eyewitness evidence, is called circumstantial. Such evidence
is things like objects (the murder weapon, clothing, etc.) or documents. The party introducing
circumstantial evidence must establish what it is. For instance, the police officer who
found the murder weapon will be called to the stand to identify it. A document must be
authenticated. The best way to do this is through the testimony of the writer or someone
who signed or witnessed the document.
How do we know if evidence is admissible? Generally, evidence is admissible if it is
material and relevant to some issue in the case. There are exceptions to this. As we
saw above, hearsay evidence is inadmissible. This is true even if it sometimes seems
relevant.
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