When you say hard evidence, I think you mean direct evidence. Wikipedia has a good summary of why circumstantial evidence is just as valid as hard evidence:

A popular misconception is that circumstantial evidence is less valid or less important than direct evidence,[2][3] which is popularly assumed to be the most powerful, but this is not the case.[4] Many successful criminal prosecutions rely largely or entirely on circumstantial evidence, and civil charges are frequently based on circumstantial or indirect evidence. Indeed, the common metaphor for the strongest possible evidence in any case—the "smoking gun"—is an example of proof based on circumstantial evidence.[5] Similarly, fingerprint evidence, videotapes, sound recordings, photographs, and many other examples of physical evidence that support the drawing of an inference, i.e., circumstantial evidence, are considered very strong possible evidence.

In practice, circumstantial evidence can have an advantage over direct evidence in that it can come from multiple sources that check and reinforce each other.[6] Eyewitness testimony can be inaccurate at times,[7] and many persons have been convicted on the basis of perjured or otherwise mistaken testimony.[8] Thus, strong circumstantial evidence can provide a more reliable basis for a verdict. Circumstantial evidence normally requires a witness, such as the police officer who found the evidence, or an expert who examined it, to lay the foundation for its admission. This witness, sometimes known as the sponsor or the authenticating witness, is giving direct (eyewitness) testimony, and could present credibility problems in the same way that any eyewitness does.