Protestant writers, indeed, have sometimes appealed to _common consent_ as a collateral proof, auxiliary to that which is more direct and conclusive; but they have done so merely because they regarded it as a _part of the evidence_, well fitted to prove what Dr.Cudworth calls "the naturality of the idea of God," and not because they confounded it with the _faculty_ by which alone that evidence can be discerned and appreciated.

They never regarded it as the sole ground of certainty either in matters of Philosophy or Faith.

Nor can it be so considered by any thoughtful mind.
For how can I be more assured of an _impersonal reason_ than of my own?
How can I be more certain of the existence and the traditions of other men, than of the facts of my own consciousness, and the spontaneous convictions of my own understanding?
or how can I be assured that, in passing from the impersonal reason to the individual mind, from the generic reason to the personal, the truth may not contract some taint of weakness or impurity from the vessel in which it is ultimately contained,--from the finite faculties by which alone it is apprehended and believed?
The fact is that any attempt to prove the truth of our faculties must necessarily fail.

Did we set ourselves to the task of proving by argument or by authority that we are not wrong in believing in our own existence or that of an external world, or did we attempt to establish the trustworthiness of our faculties by resolving it into the veracity of God, our effort must needs be as abortive as it is superfluous, since it involves the necessity not only of proving the fact, but of _proving the proof itself_, and that, too, by the aid of the very faculties whose trustworthiness is in question! There are certain ultimate facts beyond which it is impossible to push our speculative inquiries; certain first or fundamental principles of Reason, which are in themselves indemonstrable, but which constitute the ground or condition of all demonstration; certain intuitive perceptions, which are widely different from rational deductions, but which determine and govern every process of reasoning and every form of belief.

To deny the _certainty_ of our intuitive perceptions, merely because we cannot prove by argument the truth of our mental faculties, would virtually amount to a rejection of all evidence except such as comes to us only through _one_ channel, and _that_ the circuitous one of a process of reasoning; while, by the constitution of our nature, we are qualified and privileged to draw it fresh, in many cases, at its spring and fountain-head.