A tangible item of evidence can usually only have a tendency to prove or disprove a material fact. Seldom is there a "smoking gun" that indisputably proves anything. Let's look at an example. Suppose we are trying to establish that the defendant committed a homicide. A material fact would be placing the defendant at the scene of the crime. DNA evidence could tend to prove that a person had been present at the scene, from a hair, for example, or some scraped off skin. However, we generally say that something only tends to establish, not that it does establish, since the defense can argue that someone else planted the DNA evidence. Furthermore, such evidence would only tend to establish the defendant's presence, not his or her presence at the time the crime was committed, which is of course, a rather important material fact.
Eyewitness evidence is an entirely different story, tending to establish far less than people like to think. Because each of us is unique, we each "see" something entirely different, and subsequently, our memories select or recall in an extremely unreliable way. The more we understand about the human mind, the more we discover how completely unreliable eyewitness testimony is.