A fortiori is a common
Latin phrase in
philosophy which literally means "from the stronger." The phrase is commonly used to mean something like "even more so." One might say we are sometimes deceived by tricks of light, so we should not always unquestioningly accept what we think we see as truth,
a fortiori we can not trust absolutely the testimony of others, since they could also be so deceived, but beyond that they may have some reason to lie.