Stephen Toulmin argues that any claims should be based on grounds, and that the
validly claims are by virtue of the three types of warrants behind the messages:
substantive warrants motivational warrants, and authoritative warrants. Having said
that, from the point of metaperspective, then why these three types of warrants
work? In short, the validity of the warrants by virtue of what? This article is to
explore the issue. In this regard, the researcher uses the eighteen open letters that
appear on the google website in Taiwan from 2015 to 2017 as research objects, as
well as adopting the rhetorician Weaver's value arguments and Bitzer’s perspectives
of rhetorical situation to describe why these three types of warrants work? The
final analysis finds that the three types of argument warrants are based on whether
the rhetor can understand the contextual characteristics of the situation and the
overall cultural value.