“It’s not what you know, it’s what you can prove.”

The famous move quote is often repeated by litigators explaining to clients the need to build a case or a defense.  That a litigant knows certain material facts, including technical facts, to be true, does not matter.  What matters is the ability to present the facts, within the rules of evidence, and persuasively, to a trier. 

Washington’s rules of evidence, which are modeled after the federal rules, guide how evidence can, and cannot, be presented.  A comprehensive discussion of the rules is beyond the scope of this post.  The purpose of the post, rather, is to re-iterate the foundational rules for building a case that are repeated by this author in so many client discussions.

First, witnesses, generally, must have personal knowledge.  ER 602.  Any fact or exhibit sought to be to be introduced must be accompanied by testimony from someone with personal knowledge.  And persuasion is always part of the equation.  If there is conflicting evidence, corroboration is paramount.

Second, and related to the personal knowledge requirement, are the rules against hearsay.  Hearsay is an out-of-court statement offered to prove the truth of the matter asserted.  ER 801.  And hearsay is generally inadmissible.  ER 802.  The limitation is intuitive; hearsay does not come from personal knowledge of the underlying fact, and a witness or exhibit offering hearsay cannot be cross-examined on the underlying fact.

Finally, the rules do not just limit, they also give, with the prime example being the allowance of expert testimony.  A qualified expert can give testimony and opinions as to technical facts, including opinions on ultimate issues of fact.  ER 702; 704.  Cases with technical disputes, which a lay trier will not understand without assistance, almost always require expert testimony to prevail. 

In sum, it's not what you, the litigant, knows, rather it is what you can prove from witnesses with personal knowledge and qualified experts.  And the persuasive or probative value of the evidence is just as important as its admissibility.  No matter how strongly a litigant feels about his case or defense, it is unlikely to succeed unless it can be built within this framework.