1. We possess all our knowledge in the form of judgments. We know something only when we state, at least mentally, that two ideas or concepts, one of which is called the subject, the other the predicate, agree with each other, or do not agree with each other. If I say: "Shakespeare is a dramatist," I assert the agreement of the two concepts "Shakespeare" and "dramatist." If I say: "Men are not angels," I assert the disagreement of the two concepts "men" and "angels".

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2. If our judgment is in harmony with reality, it is a true judgment, as "Heat expands iron"; if it is not in harmony with reality, it is a false judgment, as "A circle is not round." If our judgments are uttered without fear of error, they are certain; if they are uttered with fear of error, they are uncertain, and therefore mere opinions. "Twice two is four," is a certain judgment; "Tomorrow will be rainy," is nothing but an opinion.

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3. Before making a judgment we must have a reason for doing so. We get our reasons for forming our judgments from what are called the sources of knowledge. There are of various kinds:

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a) Some truths are in themselves so evident as to be clearly understood by all who have the use of reason, as soon as they are put in words. Such truths are called self-evident, because they require no demonstration. For example, when once we understand the meaning of the terms, we cannot fail to grasp that "the whole is greater than its part," or that "the radii of a circle are equal," or that "everything which begins to exist must have been brought into existence by something distinct from itself," or that "what is, is, and cannot at the same time not be."

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b) From these self-evident and necessary truths another class of truths is drawn by a process of reasoning, that is, not by comparing two concepts directly with each other, but by comparing each with a third, on the principle that two things which are equal to the same things are equal to each other.

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c) Other things we know to be true from the evidence of our senses. We can trust our senses unhesitatingly if the sense we are using, e.g., sight or hearing, is in a normal condition and properly applied to the object.

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d) Lastly, there are many things which we do not know of ourselves, but which we accept on the authority of other people. If our belief  rests on the testimony of man, who can err, it is human faith; if it rests on the testimony of God, who cannot err, it is Divine Faith.

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