The Evidence
We would all probably like to believe that the people we argue with will accept our claims and reasons as perfect and complete by themselves, but most readers are unlikely to do that. They want evidence of some sort--facts, examples, statistics, expert testimony, among others--to back up our reasons. If this level of the house of cards is either unstable or absent, neither of the two levels it supports (the reasons and claim) can be effective.
To be believable and convincing, evidence should satisfy three conditions. It should be sufficient, credible , and accurate.