(From the 1847 edition)

Before we proceed further in these investigations, I must call your attention to a subject that properly belongs at the beginning of this course of study, and which will be found there should these lectures ever be published in their proper order: I allude to the various classes of truths to come under consideration in this course of instruction, with the manner in which we arrive at a knowledge or belief of them. 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 purpose, into truths that need no proof, nd truths that need proof. The human mind is so constituted that by virtue of its own laws it necessarily perceives, recognizes, or knows some truths without testimony from without. It takes direct cognizance of them, and cannot but do so.

The first class, that is, truths that need no proof, may be subdivided into truths of the pure reason and truths of sensation. These two classes are in some sense self-evident, but not in the same sense. Truths of the pure reason are intuitions of that faculty, and truths of sensation are intuitions of the senses. I shall therefore speak of self-evident truths of reason and self-evident truths of sensation. I must assume that you possess some knowledge of psychology, and take it for granted that you understand the difference between the intuitions of reason and the intuitions of sense.

By self-evident truths of reason, then, I mean that class of truths that are directly intuited and affirmed by that faculty in the light of their own evidence, and by virtue of its own laws, whenever they are so stated that the terms of the proposition in which they are conveyed are understood. They are not arrived at by reasoning, or by evidence of any kind except what they have in themselves. As soon as the terms of the propositions in which they are stated are understood, the reason instantly and positively affirms their truth. It is unnecessary and preposterous to attempt any other proof of this class of truths than to frame a perspicuous statement of them. Nay, it is positively injurious, because absurd, to attempt to prove in the common acceptation of the term prove a self-evident truth of reason. All attempts to prove such truths by reasoning involve an absurdity, and are as much a work of supererogation as it would be to attempt to prove that you see an object with your eyes fully open and set upon it.

The mathematical axioms belong to this class.

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. Although this class of truths are never arrived at by reasoning, yet much use is made of them in reasoning, since the major premise of a syllogism is often a self-evident truth of reason.

This class of truths is affirmed by a faculty entirely distinct from the understanding, or that power that gains all its knowledge from sense. It takes cognizance of a class of truths that from their nature forever lie concealed from the senses and consequently from the understanding. Sensation can never give us the abstract truths of mathematics. It can never give us the absolute or the infinite. It cannot give moral law or law at all. Sensation can give facts, but not laws and principles.

That God and space and duration are infinite, that all God's attributes must be infinite, are self-evident truths of reason; that is, they are truths of a priori affirmation and assumption. They are never arrived at by reasoning, or by induction, and never can be. The mind only knows them by virtue of its own laws, and directly assumes and intuits them whenever they are suggested. The eye of reason sees them as distinctly as the mind sees objects of vision presented to the fleshly organ of vision. The mind is so constructed that it sees some things with the natural fleshy eye, and some truths it sees directly with its own eye without the use of an eye of flesh. All the self-evident truths of reason belong to this class; that is, they are truths which the mind sees and knows, and does not merely believe. In reasoning, the bare statement of a self-evident truth is enough, provided, as has been said, that it is so perspicuously stated that the terms of the proposition are understood. It should be borne in mind, in reasoning, that all men have minds, and that the laws of knowledge are physical, and, of course, fixed and common to all men. The conditions of knowledge are in all men the same. We are therefore always to assume that self-evident truths cannot but be known as soon as they are stated with such perspicuity as that the terms in which they are expressed are understood. Our future inquiries will present many illustrations of the truth of these remarks.

It should be also remarked that universality is an attribute of the self-evident truths of reason. That is, they are universal in the sense:

1. That all men affirm them to be true when they understand them.

2. They all affirm them to be true in the same way; that is, by direct intuition. Or they perceive them in their own light, and not through the medium of reasoning, demonstration, or sense.

3. Self-evident truths of reason are true without exception, and in this sense also universal.

4. Necessity is also an attribute of self-evident truths. That is, they are necessarily true and cannot but be so regarded. And when the conditions which have been named are fulfilled, they cannot but be so known to every moral agent.

Self-evident truths of reason may be again divided into truths merely self-evident, and first-truths of reason. This class of truths possess all the characteristics of self-evident truths, to wit: they are universal truths; they are necessary truths; they are truths of direct intuition; they are truths of certain knowledge.

Their peculiarity is this: they are truths that are necessarily and universally known by moral agents. That is, they are not distinguished from mere self-evident truths of reason, except by the fact that from the laws of moral agency they are known universally, and all moral agents do and must possess certain knowledge of them.

They are truths of necessary and universal assumption. Whether they are at all times, or at any time, directly thought of or made the particular object of the mind's attention or not, they are nevertheless at all times assumed by a law of universal necessity. Suppose, for example, that the law of causality should not be at all times or at any time a subject of distinct thought and attention. Suppose that the proposition in words should never be in the mind, that "every event must have a cause." Still, the truth is there in the form of absolute knowledge, a necessary assumption, an a priori affirmation, and the mind has so firm a hold of it as to be utterly unable to overlook, forget, or practically deny it.

Every mind has it as a certain knowledge long before it can understand the language in which it is expressed, and no statement or evidence whatever can give the mind any firmer conviction of its truth than it had from necessity at first. This is true of all the truths of this class. They are always and necessarily assumed by all moral agents whether distinctly thought of or not. And for the most part this class of truths are assumed without being frequently, or at least, without being generally the object of thought or direct attention. The mind assumes them without a direct consciousness of the assumption.

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.

But it is proper to inquire whether there are any conditions of this assumption, and if so, what they are? Does the intelligence make this assumption upon certain conditions, or independent of all or any conditions? The true answer to this inquiry is that the mind makes the assumption only upon the fulfillment of certain conditions. These conditions being fulfilled, the intelligence instantly and necessarily makes the assumption by a law of its own nature, and makes it whether the assumption be a distinct object of consciousness or not.

The only condition of this assumption that needs to be mentioned is the perception of that by the mind to which the first truth sustains the relation of a logical antecedent or of a logical condition. For example, to develop and necessitate the assumption that every event must have a cause, the mind only needs to perceive or to have the conception of an event, whereupon the assumption in question instantly follows by a law of the intelligence. This assumption is not a logical deduction from any premise whatever, but upon the perception of an event, or upon the mind's having the idea or notion of an event, the intelligence irresistibly, by virtue of its own laws, assumes the first-truth of causality as the logical and necessary condition of the event; that is, it assumes that an event and every event must have a cause.