In the U.S. American trial system proof mainly consists of live witnesses presented in open court under oath before the judge, jury, and parties, subject to perjury laws. Cross-examination of the witnesses in that setting is the principal (though not the only) form of testing their reliability. It is for these reasons that we have a rule against hearsay (second-hand reporting in court of what someone has said outside of court).
Scholarly Commons Citation
Rothstein, Paul F., "Some Thoughts on the Fundamentals of an Evidence Code From the U.S. American Perspective" (2014). Georgetown Law Faculty Publications and Other Works. Paper 1410.