Sforno on Deuteronomy 13:3/ספורנו על דברים י״ג:ג׳

1אשר לא ידעתם, the miracle proving the existence of such a phenomenon seeing you had never heard of its existence. Seeing we do not know the function of the phenomenon makes it more difficult to dismiss the idea that it possesses powers. It is unlike sun and moon whose functions and limitations we know. We know that it is useless to pray to sun and moon, as their obvious limitations are known to us.