The sources of evidence have the different witnesses attached to them. The primary source of evidence is to prove a case in the court of law. In other words, the source of the evidence is to make sure that there is sufficient information to build a case. Thus, the evidence can come in many forms - the physical, the document, the demonstrative, and the testimony. However, the physical evidence is the most reliable form. In an unchanged state, the physical evidence, like DNA is very reliable.
