The question is not whether reason is the instrument by which or the medium through which we can be drawn to faith. For we acknowledge that reason can be both: the former indeed always and everywhere; the later with regard to presupposed articles. Rather the question is whether it is the first principle from which the doctrines of faith are proved; or the foundation upon which they are built, so that we must hold to be false in things of faith what the nature light or human reason cannot comprehend. This we deny.
…If reason is the principle of faith, then first it would follow that all religion is natural and demonstrable by natural reason and natural light. Thus nature and grace, natural and supernatural revelation would be confounded.
…A ministerial and organic relation is quite different from a principial and despotic.
…We must observe the distinction between an instrument of faith and the foundation of faith.
…The Lutherans falsely object to us that we hold reason to be the principle and rule of demonstration in controversies because we sometimes draw arguments from reason, and argue from reason against the ubiquity of Christ’s body. For we assign to reason only a ministerial and instrumental, not a principal office. And if, in compound questions, we use reason for the purpose of proof, it bears the relation not of a principle but of means from which the theologian argues; and the are not with us primary arguments, but only secondary and auxiliary forces. Besides, while the theologian uses arguments drawn from reason, he does it rather as a philosopher rather than as a theologian. As to the ubiquity of the body of Christ, we reject this doctrine both philosophically and theologically, because it is absurd and contradicts the first principles of theology and philosophy.
(cited as Francis Turretin, Institutes of Elenctic Theology, 1.8.4, 5, 6, 24 at this link)