Definitions: (1) capable of being believed; trustworthy; (2) worthy of confidence; reliable
• To be persuasive we must be believable; to be believable we must be credible; to be credible we must be truthful. — Edward Roscoe Murrow [born Egbert Roscoe Murrow] (1908–1965) American broadcast journalist
• A statement is persuasive and credible either because it is directly self-evident or because it appears to be proved from other statements that are so. — Aristotle (384-322 bc) Greek Philosopher
Comment: Conventional credentials – diplomas, titles, lists of accomplishments, authorship, family ties, and children – give a person credibility. No matter what a person says they are going to do, who they are is evident in what they have done.
Somebody might say he or she believes in certain positive qualities, and these intentions are valuable, but who they actually are and what they have in fact done, are a good measure of how well they will live up to their ideals.