The Mesilat Yesharim starts off:
יסוד החסידות ושרש העבודה התמימה הוא שיתברר ויתאמת אצל האדם מה חובתו
(Translation: The foundation of saintliness and the root of perfection in the service of God lies in a man's coming to see clearly and to recognize as a truth the nature of his duty in the world and the end towards which he should direct his vision and his aspiration in all of his labors all the days of his life.)
the words שיתברר ויתאמת are translated here as "coming to see clearly and to recognize as a truth".
what does it mean to come to "recognize as a truth"?
does this mean to go into chakira (rational inquiry/logical proofs) to try to arrive at the truth as to the existence of God and the divine origin of the torah?
if no, how else can something be recognized as a truth?
you can't say he means belief because it would not be accurate to say "recognize as a truth", and we assume that each word of this book was carefully chosen (as is the way of the torah sages).