The apostle Paul, in Romans 1 and 2, explains that God has made each one of us with an innate knowledge of Him and of right and wrong. We don’t grow up in a vacuum; there are some things we come to know because He has placed within us the ability to grasp them even without the aid of divine revelation. Paul called them the law written on the heart; the Founders of America referred to them as self-evident truths.
Charles Finney, in his Systematic Theology, makes the same point, albeit in his own way:
All human investigations proceed upon the assumption of the existence and validity of our faculties, and that their unequivocal testimony may be relied upon. To deny this is to set aside at once the possibility of knowledge or rational belief, and to give up the mind to universal skepticism. The classes of truths to which we shall be called upon to attend in our investigations may be divided, with sufficient accuracy for our purposes, into truths that need no proof, and truths that need proof. . . .
The self-evident truths of reason are truths of certain knowledge. When once so stated, or in any way presented to the mind as to be understood, the mind does not merely believe them, it knows them to be absolutely true. That is, it perceives them to be absolute truths, and knows that it is impossible that they should not be true. . . .
For example, we act every moment, judge, reason, and believe, upon the assumption that every event must have a cause, and yet we are not conscious of thinking of this truth, nor that we assume it, until something calls the attention to it. First-truths of reason, then, let it be distinctly remembered, are always and necessarily assumed though they may be seldom thought of. They are universally known before the words are understood by which they may be expressed, and although they may never be expressed in a formal proposition, yet the mind has as certain a knowledge of them as it has of its own existence.