Front Page Titles (by Subject) § 10.: First Principles of Necessary Truths - Selections from the Scottish Philosophy of Common Sense
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§ 10.: First Principles of Necessary Truths - Thomas Reid, Selections from the Scottish Philosophy of Common Sense 
Selections from the Scottish Philosophy of Common Sense, edited, with an introduction by G.A. Johnston (Chicago: Open Court, 1915).
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First Principles of Necessary Truths
About most of the first principles of necessary truths there has been no dispute, and therefore it is the less necessary to dwell upon them. It will be sufficient to divide them into different classes; to mention some, by way of specimen, in each class; and to make some remarks on those of which the truth has been called in question.
They may, I think, most properly be divided according to the sciences to which they belong.
1. There are some first principles that may be called grammatical: such as, That every adjective in a sentence must belong to some substantive expressed or understood; that every complete sentence must have a verb.
2. There are logical axioms: such as, That any contexture of words, which does not make a proposition, is neither true nor false; That every proposition is either true or false; That no proposition can be both true and false at the same time; That reasoning in a circle proves nothing; That whatever may be truly affirmed of a genus, may be truly affirmed of all the species, and all the individuals belonging to that genus.
3. Every one knows there are mathematical axioms. Mathematicians have, from the days of Euclid, very wisely laid down the axioms or first principles on which they reason. And the effect which this appears to have had upon the stability and happy progress of this science, gives no small encouragement to attempt to lay the foundation of other sciences in a similar manner, as far as we are able.
4. I think there are axioms, even in matters of taste. Notwithstanding the variety found among men in taste, there are, I apprehend, some common principles, even in matters of this kind. I never heard of any man who thought it a beauty in a human face to want a nose, or an eye, or to have the mouth on one side.
That an unjust action has more demerit than an ungenerous one: That a generous action has more merit than a merely just one: That no man ought to be blamed for what it was not in his power to hinder: That we ought not to do to others what we would think unjust or unfair to be done to us in like circumstances. These are moral axioms, and many others might be named which appear to me to have no less evidence than those of mathematics.
Some perhaps may think that our determinations, either in matters of taste or in morals, ought not to be accounted necessary truths: That they are grounded upon the constitution of that faculty which we call taste, and of that which we call the moral sense or conscience; which faculties might have been so constituted as to have given determinations different, or even contrary to those they now give: That, as there is nothing sweet or bitter in itself, but according as it agrees or disagrees with the external sense called taste; so there is nothing beautiful or ugly in itself, but according as it agrees or disagrees with the internal sense, which we also call taste; and nothing morally good or ill in itself, but according as it agrees or disagrees with our moral sense.
This indeed is a system, with regard to morals and taste, which hath been supported in modern times by great authorities. And if this system be true, the consequence must be, that there can be no principles, either of taste or of morals, that are necessary truths. For, according to this system, all our determinations, both with regard to matters of taste, and with regard to morals, are reduced to matters of fact—I mean to such as these, that by our constitution we have on such occasions certain agreeable feelings, and on other occasions certain disagreeable feelings.
But I cannot help being of a contrary opinion, being persuaded that a man who determined that polite behaviour has great deformity, and that there is great beauty in rudeness and ill-breeding, would judge wrong, whatever his feelings were.
In like manner, I cannot help thinking that a man who determined that there is more moral worth in cruelty, perfidy, and injustice, than in generosity, justice, prudence, and temperance, would judge wrong, whatever his constitution was.
And, if it be true that there is judgment in our determinations of taste and of morals, it must be granted that what is true or false in morals, or in matters of taste, is necessarily so. For this reason, I have ranked the first principles of morals and of taste under the class of necessary truths.
6. The last class of first principles I shall mention, we may call metaphysical.
I shall particularly consider three of these, because they have been called in question by Mr Hume.
The first is, That the qualities which we perceive by our senses must have a subject, which we call body, and that the thoughts we are conscious of must have a subject, which we call mind.
The second metaphysical principle I mention is—That whatever begins to exist, must have a cause which produced it.
The last metaphysical principle I mention, which is opposed by the same author, is, That design and intelligence in the cause may be inferred, with certainty, from marks or signs of it in the effect.1
[1 ]Ibid., pp. 452-457.