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.

The condition upon which the first-truths of reason are assumed or developed is called the chronological condition of their development, because it is prior in time and in the order of nature to their development. The mind perceives an event. It thereupon assumes the first-truth of causality. It perceives body, and thereupon assumes the first-truth, space is and must be. These first-truths, let it be repeated, are not assumed in the form of a proposition, thought of or expressed in words, nor is the mind at the time always, or perhaps ever, at first, distinctly conscious of the assumption, yet the truth is from that moment within the mind's inalienable possession, and must forever after be recognized in all the practical judgments of the mind.

Thus, it should be distinctly said, the first-truths of reason lie so deep in the mind as perhaps seldom to appear directly on the field of conscious thought, and yet so absolutely does the mind know them that it can no more forget, overlook, or practically deny them, than it can forget, overlook, or in practice deny its own existence.

I have said that all reasoning proceeds upon the assumption of these truths. It must do so of necessity. It is preposterous to attempt to prove first-truths to a moral agent:

for if a moral agent, he must absolutely know them already, and if he did not, in no possible way could he be put in possession of them except by presenting to his perception the chronological condition of their development, and in no case could any thing else be needed, for upon the occurrence of this perception, the assumption or development follows by a law of absolute and universal necessity. And until these truths are actually developed, no being can be a moral agent.

There is no reasoning with one who calls in question the first-truths of reason, and demands proof of them. All reasoning must, from the nature of mind and the laws of reasoning, assume the first-truths of reason as certain, and admitted, and as the a priori condition of all logical deductions and demonstrations. Some one of these must be assumed as true, directly or indirectly, in every syllogism and in every demonstration.

In all our future investigations in the line of truth we shall pursue, we shall have abundant occasions for the application and illustration of what has now been said of first-truths of reason. If, at any stage of our progress, we light upon a truth of this class, let it be borne in mind that the nature of the truth is the preclusion, or as lawyers would express it, the estoppel of all controversy. To deny the reality of this class of truths is to deny the validity of our most perfect knowledge and of course it is a denial of the validity of our faculties. The only question to be settled in respect to this class of truths, is, does the truth in question belong to this class? There are many of this class that have not been generally recognized as belonging to it. Of this we shall have abundant instances fall in our way as we proceed in our investigations. There are many truths which men, all sane men, certainly know, of which they not only seldom think, but which, in theory, they strenuously deny.

Before I dismiss this branch of our subject, I will mention some of the many truths that undeniably belong to this class, leaving others to be mentioned as we proceed and fall in with them in future investigations.

I have already noticed three of this class, to wit; the truth of causality the existence of space and of time. That the whole of any thing is equal to all its parts, is also a truth of this class, universally and necessarily known and assumed by every moral agent. Also, that a thing cannot be and not be at the same time.

A third class of self-evident truths are particular truths of reason. The reason directly intuits and affirms them. They are truths of certain knowledge, but have not the attributes of universality or infinity. To this class belong the truths of our own existence, of personal identity, and individuality. These are not truths of sensation, nor are they first or self-evident truths according to the common use of those terms. Yet they are truths of rational intuition, and are seen to be true in the light of their own evidence, and as such are given to us as undoubtable verities by consciousness.

All the truths that come within the pale of our own experience, that is, all our mental exercises and states are truths self-evident to us. We need no proof of them. Whether they are phenomena or states of the Intellect, of the Will, or of the Sensibility. When thus spoken of, in mass, they cannot be called self-evident truths except in the sense that to ourselves they appear on the field of consciousness as facts or realities, and we know or affirm them with undoubting certainty.

Truths of sensation, I have said, are in a certain sense self-evident truths. That is, they are facts of which the mind has direct knowledge through the medium of the senses. In speaking of truths of sensation as in some sense self-evident, I mean of course truths or facts of our own senses, or those revealed directly to us by our senses. I know it is not common to speak of this class of truths as self-evident; and they are not so in the sense in which simple rational intuitions are. Yet they are facts or truths which need no proof to establish them to us. The fact that I hold this pen in my hand is as really self-evident to me as that three and two are five. I as really know or perceive the one as the other, and neither the one nor the other needs any proof. It is not my design to exhaust this subject, nor to enter upon nice and highly metaphysical distinctions, but only to give hints and make suggestions that will call your attention to the subject, and meet our necessities during our present course of study, leaving it to your convenience to enter upon a more critical analysis of this subject.

Of truths that require proof, the first class to which I must call attention is the truths of demonstration. This class of truths admit of so high a degree of proof that when the demonstration is complete, the intelligence affirms that it is impossible that they should not be true. This class when truly demonstrated, are known to be true with no less certainty than self-evident truths: but the mind arrives not at the perception and knowledge of them in the same way. That class is arrived at universally, directly and a priori, that is, by direct intuition without reasoning. This class is arrived at universally by reasoning. The former are obtained without any logical processes, while this last class is always and necessarily obtained as a result of a logical process. We often get these truths by a process strictly logical without being at all aware of the way in which we came to be possessed of them. This class, then, unlike the other, are not to be communicated and established without reasoning, but by reasoning. In this class of truths the mind from its own laws will not rest unless they be demonstrated. They admit of demonstration, and from their nature and the nature of the intelligence, they must be demonstrated before they can be known and rested in as certain knowledge. Many of them may be received in the sense of being believed without an absolute demonstration. But the mind cannot properly be said to know them until it has gone through with the demonstration, and then it cannot but know them.

To possess the mind of a first-truth of reason you need only to present the chronological condition of its development. To reveal a self-evident truth of reason, you need only to state it in terms of sufficient perspicuity. But to prove a truth belonging to the class now under consideration you must fulfill the logical conditions of the intellect's affirming it. That is, you must demonstrate it.

The next class to be considered are truths of revelation. I mean truths revealed by divine Inspiration. All truths are in some way revealed to the mind, but not all by the Inspiration of the Holy Spirit. Some of this class are known and some only believed by the mind. That is, some of these truths are objects or truths of knowledge or of intuition when brought by the Holy Spirit within the field of vision or intuition. Others of them are only truths of faith or truths to be believed. The divinity of the Lord Jesus Christ is a truth of revelation of the first class, that is, a truth of intuition or of certain knowledge when revealed to the mind by the Holy Spirit. This truth, when thus revealed, the pure reason directly intuits. It knows that Jesus is the true God and eternal life by the same law by which it knows the first-truths of reason. The only account the soul can give of this truth is, that it knows it to be true. It sees or perceives it to be true. But this perception or intuition is conditionated upon the revelation of the Holy Spirit. "He shall take of mine," said Jesus, "and shew it unto you." More on this topic in its proper place. The facts and truths connected with the humanity of the Lord Jesus are of the second class of truths of revelation, that is, they are only truths of belief or of faith, as distinct from truths of the pure reason or of intuition.

This class of truths, from their nature, are not susceptible of intuition. They may be so revealed that the soul will have no doubt of them, and hardly distinguish them from truths of certain knowledge; nevertheless, they are only believed and not certainly known as truths of intuition are.

The Bible is not of itself, strictly and properly a revelation to man. It is, properly speaking, rather a history of revelations formerly made to certain men. To be a revelation to us, its truths must be brought by the Holy Spirit within the field of spiritual vision. This is the condition of our either knowing or properly believing the truths of revelation. `No man can say that Jesus is the Lord, but by the Holy Spirit.' `No man can come to me, except the Father which hath sent me, draw him.' `They shall all be taught of God.' `The natural man receiveth not the things of the Spirit of God, neither can he know them, because they are spiritually discerned.' `He that is spiritual [has the Spirit,] judgeth all things.'

But I must not in this place dwell longer upon this subject. I would only add now that those who call in question the divinity of Christ exhibit conclusive evidence that Christ has never been revealed to them by the Holy Spirit. Those who hold his divinity as a theory or opinion are not at all benefitted by it, for Christ is not savingly known to any except by the revelation of the Holy Spirit.

To the classes of truths already considered might be added several others, such as Probable Truths, Possible Truths, etc.

But I have carried this discussion far enough to answer the purposes of this course of instruction, and I trust far enough to impress your minds with a sense of the importance of attending to the classifying of truths and of ascertaining the particular class to which a truth belongs as the condition of successfully attempting to gain the possession of it yourself, or of possessing the minds of others with it. As religious teachers you cannot be too deeply impressed with the importance of attending to this classification. I am fully convinced that much of the inefficiency of religious teachers is owing to the fact that they do not sufficiently study and comply with the laws of knowledge and belief to carry conviction to the minds of their hearers. They seem not to have considered the different classes of truths, and how the mind comes to possess a knowledge or belief of them. Consequently they either spend time in worse than useless efforts to prove first or self-evident truths, or expect truths susceptible of demonstration to be received and rested in without such demonstration. They often make little or no distinction between the different classes of truths, and seldom or never call the attention of their hearers to this distinction. Consequently, they confuse and often confound their hearers by gross violations of all the laws of logic, knowledge, and belief. I have often been pained and even agonized at the faultiness of religious teachers in this respect. Study to show yourselves approved, workmen that need not be ashamed, and able to commend yourselves to every man's conscience in the sight of God.