(From the 1851 edition)

All teaching and reasoning take certain truths as granted. That the unequivocal, a priori affirmations of the reason are valid, for all the truths and principles thus affirmed, must be assumed and admitted; or every attempt to construct a science of any kind, or to attain to certain knowledge upon any subject, is vain and even preposterous. As I must commence my lectures on moral government by laying down certain moral postulates, or axioms, which are, a priori, affirmed by the reason, and therefore self-evident to all men when so stated as to be understood, I will spend a few moments in stating certain facts belonging more appropriately to the department of psychology. Theology is so related to psychology that the successful study of the former without a knowledge of the latter is impossible. Every theological system and every theological opinion assumes something as true in psychology. Theology is, to a great extent, the science of mind in its relations to moral law. God is a mind or spirit: all moral agents are in his image. Theology is the doctrine of God, comprehending His existence, attributes, relations, character, works, word, government (providential and moral), and, of course, it must embrace the facts of human nature and the science of moral agency. All theologians do and must assume the truth of some system of psychology and mental philosophy, and those who exclaim most loudly against metaphysics no less than others.

There is a distinction between the mind's knowing the truth and knowing that it knows it. Hence I begin by defining self-consciousness.

Self-consciousness is the mind's recognition of itself. It is the noticing of, or act of knowing, itself: its existence, attributes, acts, and states, with the attributes of liberty or necessity which characterize those acts and states. Of this, I shall frequently speak hereafter.

The revelations of self-consciousness

Self-consciousness reveals to us three primary faculties of mind which we call intellect, sensibility, and will. The intellect is the faculty of knowledge; the sensibility is the faculty or susceptibility of feeling; the will is the executive faculty, or the faculty of doing or acting. All thinking, perceiving, intuiting, reasoning, opining, forming notions or ideas, belong to the intellect.

Consciousness reveals the various functions of the intellect, and also of the sensibility and will. In this place, we shall attend only to the functions of the intellect, as our present business is to ascertain the methods by which the intellect arrives at its knowledges, which are given to us in self-consciousness.

Self-consciousness is, itself, of course, one of the functions of the intellect; and here it is in place to say that a revelation in consciousness is science and knowledge. What consciousness gives us we know. Its testimony is infallible and conclusive upon all subjects upon which it testifies.

Among other functions of the intellect, which I need not name, self-consciousness reveals the three-fold fundamental distinction of the sense, the reason, and the understanding.

Of the sense

The sense is the power that perceives sensation and brings it within the field of consciousness. Sensation is an impression made upon the sensibility by some object without, or some thought within the mind. The sense takes up, or perceives the sensation, and this perceived sensation is revealed in consciousness. If the sensation is from some object without the mind, as sound or color, the perception of it belongs to the outer sense. If from some thought, or mental exercise, the perception is of the inner sense. I have said that the testimony of consciousness is conclusive for all the facts given by its unequivocal testimony. We neither need, nor can we have, any higher evidence of the existence of a sensation than is given by consciousness.

Our first impressions, thoughts, and knowledge, are derived from sense. But knowledge derived purely from this source would, of necessity, be very limited.

Of the reason

Self-consciousness also reveals to us the reason or the a priori function of the intellect. The reason is that function of the intellect which immediately holds or intuits a class of truths which, from their nature, are not cognizable either by the understanding or the sense. Such, for example, is the mathematical, philosophical, and moral axioms and postulates. The reason gives laws and first principles. It gives the abstract, the necessary, the absolute, the infinite. It gives all its affirmations by a direct beholding or intuition, and not by induction or reasoning. The classes of truths given by this function of the intellect are self-evident. That is, the reason intuits or directly beholds them, as the faculty of sense intuits or directly beholds a sensation. Sense gives to consciousness the direct vision of a sensation, and therefore the existence of the sensation is certainly known to us. The reason gives to consciousness the direct vision of the class of truths of which it takes cognizance; and of the existence and validity of these truths we can no more doubt than of the existence of our sensations.

Between knowledge derived from sense and from reason there is a difference: in one case, consciousness gives us the sensation: it may be questioned whether the perceptions of the sense are a direct beholding of the object of the sensation, and consequently whether the object really exists and is the real archetype of the sensation. That the sensation exists we are certain, but whether that exists which we suppose to be the object and the cause of the sensation admits of doubt. The question is, does the sense immediately intuit or behold the object of the sensation? The fact that the report of sense cannot always be relied upon seems to show that the perception of sense is not an immediate beholding of the object of the sensation; sensation exists, this we know, that it has a cause we know; but that we rightly know the cause or object of the sensation we may not know.