In Tartuffe, what is Moliere saying about common sense and reason?

Some of the characters in this play provide a powerful
demonstration of the dangers of lacking common sense and reason! Moliere seems to be
arguing for the need to have moderation and common sense in all things. Consider how
Tartuffe and Orgon are both presented as men of excess and obsession. Tartuffe is of
course a glutton, and Orgon becomes obsessed with Tartuffe to such an extent that he
completely loses his sense of reason and nearly ruins his own family as a result. Even
when the truth emerges and Orgon finds out that Tartuffe is actually something of a
fake, Orgon reacts in a typically extreme way, saying that he will "never" trust another
man and will implacably oppose himself against those who say they are
virtuous.

In comparison to these two characters, that
provide ample learning opportunities concerning the need for moderation and common
sense, Cleante provides the voice of reason in the play, counseling Orgon about the need
for moderation and advising him to chagne his character. Note what he says to
Orgon:

You
never are content with moderation... you fly back and forth between
extremes.

It is Cleante that
tells Orgon what conclusions he should make about the world following his friendship
with Tartuffe. He must not oppose himself to all men who appear good, but adopt
moderation in his own character, especially in the areas of judgement and behaviour, so
that he can act less compulsively. Cleante is thus the vehicle used by Moliere to
announce one of the key messages of the play: moderation and common sense are much
better than obsession or the lack of reason.