(3) First truths are truths of necessary and universal knowledge. That is, they are not merely knowable, but they are known to all moral agents by a necessary law of their intellect.

That space and time are, and must be, that every event has and must have a cause, and such like truths, are universally known and assumed by every moral agent whether the terms in which they are stated have ever been so much as heard by him or not. This last is the characteristic that distinguished first truths from others merely self-evident, of which we shall soon speak.

(4) First truths are, of course, self-evident. That is, they are universally directly beheld in the light of their own evidence.

(5) First truths are truths of the pure reason, and of course truths of certain knowledge. They are universally known with such certainty as to render it impossible for any moral agent to deny, forget, or practically overlook them. Although they may be denied in theory, they are always, and necessarily, recognized in practice.

No moral agent, for example, can, by any possibility, practically deny, or forget, or overlook the first truths that time and space exist and must exist, that every event has and must have a cause.

It is, therefore, always to be remembered that first truths are universally assumed and known, and in all our teachings, and in all our inquiries we are to take the first truths of reason for granted. It is preposterous to attempt to prove them, for the reason that we necessarily assume them as the basis and condition of all reasoning.

The mind arrives at a knowledge of these truths by directly and necessarily beholding them, upon condition of its first perceiving their logical condition. The mind beholds or attains to the conception of an event. Upon this conception it instantly assumes, whether it thinks of the assumption or not, that this event had, and that every event must have, a cause.

The mind perceives or has the notion of body. This conception necessarily develops the first truth, space is and must be.

The mind beholds or conceives of succession; and this beholding or conception necessarily develops the first truth, time is and must be.

As we proceed we shall notice divers truths which belong to this class, some of which, in theory, have been denied. Nevertheless, in their practical judgments, all men have admitted them and given as high evidence of their knowing them as they do of knowing their own existence.

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," or that this proposition should be denied. 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 or 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 distinct consciousness of the assumption. For example, we act every moment, and judge, and 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.

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, being 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 deduction and demonstration. Some one of these must be assumed as true, directly or indirectly, in every syllogism and in every demonstration.

In all our future investigations we shall have abundant occasion 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. The only question to be settled is, does the truth in question belong to this class? 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.

2. The second class of truths that need no proof are self-evident truths, possessing the attributes of necessity and universality. Of these truths, I remark:

(1) That they, like first truths, are affirmed by the pure reason, and not by the understanding, nor the sense.

(2) They are affirmed, like first truths, a priori; that is, they are directly beheld or intuited, and not attained to by evidence or induction.

(3) They are truths of universal and necessary affirmation, when so stated to be understood. By a law of the reason, all sane men must admit and affirm them in the light of their own evidence, whenever they are understood.

This class, although self-evident when presented to the mind, are not, like first truths, universally and necessarily known to all moral agents. The mathematical axioms and first principles, the a priori grounds and principles of all science, belong to this class.

(4) They are, like first truths, universal in the sense that there is no exception to them.

(5) They are necessary truths. That is, the reason affirms, not merely that they are, but that they must be, true; that these truths cannot but be. The abstract, the infinite, belong to this class.

To compel other minds to admit this class of truths, we need only to frame so perspicuous a statement of them as to cause them to be distinctly perceived or understood. This being done, all sound minds irresistibly affirm them, whether the heart is, or is not, honest enough to admit the conviction.

3. A third class of truths that need no proof are truths of rational intuition, ut possess not the attributes of universality and necessity.

Our own existence, personality, personal identity, etc, belong to this class. These truths are intuited by the reason, are self-evident, and given, as such, in consciousness; they are known to self, without proof, and cannot be doubted. They are at first developed by sensation, but not inferred from it.

Suppose a sensation to be perceived by the sense, all that could be logically inferred from this is that there is some subject of this sensation, but that I exist, and am the subject of this sensation, does not logically appear. Sensation first awakens the mind to self-consciousness; that is, a sensation of some kind first arouses the attention of the mind to the facts of its own existence and personal identity. These truths are directly beheld and affirmed. The mind does not say, I feel, or I think, and therefore I am, for this is a mere sophism; it is to assume the existence of the I as the subject of feeling, and afterwards to infer the existence of the I from the feeling or sensation.

4. A fourth class of truths that need no proof are sensations. It has been already remarked that all sensations given by consciousness are self-evident to the subject of them. Whether I ascribe my sensations to their real cause may admit of doubt, but that the sensation is real there can be no doubt. The testimony of the sense is valid for that which it immediately beholds or intuits, that is, for the reality of the sensation. The judgment may err by ascribing the sensation to the wrong cause.

But I must not proceed further with this statement; my design has been not to enter too minutely into nice metaphysical distinctions, nor by any means to exhaust the subject of this lecture, but only to fix attention upon the distinctions upon which I have insisted for the purpose of precluding all irrelevant and preposterous discussions about the validity of first and self-evident truths. I must assume that you possess some knowledge of psychology and of mental philosophy, and leave to your convenience a more thorough and extended examination of the subject but hinted at in this lecture.