The Duties of the Heart, by Rabbi Bachye, tr. by Edwin Collins, , at sacred-texts.com
The faith of the believer is not complete unless he knows the meaning and the reasons of his belief. And this knowledge that enables one to fulfil the duties of the heart, is the hidden wisdom that is the light of hearts and the bright effulgence of souls; and concerning it Scripture says:—(Ps. li. 6) "Behold Thou desirest truth in the inward parts and in the hidden, Thou wilt make me to know wisdom." It is only those of weak intellect who are not culpable if they take on trust what it is man's duty to search out. But whoever has the strength of
intellect, and the power to sift and prove, is sinful if he neglects to do so, and also even his lack of knowledge is a sin.