Front Page Titles (by Subject) § 9.: The First Principles of Contingent Truths - Selections from the Scottish Philosophy of Common Sense
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§ 9.: The First Principles of Contingent 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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The First Principles of Contingent Truths
It is necessary that the first principles of knowledge be distinguished from other truths, and presented to view, that they may be sifted and examined on all sides. In order to this end, I shall attempt a detail of those I take to be such, and of the reasons why I think them entitled to that character.
If the enumeration should appear to some redundant, to others deficient, and to others both—if things which I conceive to be first principles, should to others appear to be vulgar errors, or to be truths which derive their evidence from other truths, and therefore not first principles—in these things every man must judge for himself. I shall rejoice to see an enumeration more perfect in any or in all of those respects; being persuaded that the agreement of men of judgment and candour in first principles would be of no less consequence to the advancement of knowledge in general, than the agreement of mathematicians in the axioms of geometry has been to the advancement of that science.
The truths that fall within the compass of human knowledge, whether they be self-evident, or deduced from those that are self-evident, may be reduced to two classes. They are either necessary and immutable truths, whose contrary is impossible; or they are contingent and mutable, depending upon some effect of will and power, which had a beginning, and may have an end.
That a cone is the third part of a cylinder of the same base and the same altitude, is a necessary truth. It depends not upon the will and power of any being. It is immutably true, and the contrary impossible. That the sun is the centre about which the earth, and the other planets of our system, perform their revolutions, is a truth; but it is not a necessary truth. It depends upon the power and will of that Being who made the sun and all the planets, and who gave them those motions that seemed best to him.
As the minds of men are occupied much more about truths that are contingent than about those that are necessary, I shall first endeavour to point out the principles of the former kind.
1. First, then, I hold, as a first principle, the existence of everything of which I am conscious.
This, I think, is the only principle of common sense that has never directly been called in question. It seems to be so firmly rooted in the minds of men, as to retain its authority with the greatest sceptics. Mr Hume, after annihilating body and mind, time and space, action and causation, and even his own mind, acknowledges the reality of the thoughts, sensations, and passions of which he is conscious.
2. Another first principle, I think, is, That the thoughts of which I am conscious, are the thoughts of a being which I call myself, my mind, my person.
The thoughts and feelings of which we are conscious are continually changing, and the thought of this moment is not the thought of the last; but something which I call myself, remains under this change of thought. This self has the same relation to all the successive thoughts I am conscious of—they are all my thoughts; and every thought which is not my thought, must be the thought of some other person.
If any man asks a proof of this, I confess I can give none; there is an evidence in the proposition itself which I am unable to resist. Shall I think that thought can stand by itself without a thinking being? or that ideas can feel pleasure or pain? My nature dictates to me that it is impossible.
3. Another first principle I take to be—That those things did really happen which I distinctly remember.
This has one of the surest marks of a first principle; for no man ever pretended to prove it, and yet no man in his wits calls it in question: the testimony of memory, like that of consciousness, is immediate; it claims our assent upon its own authority.
4. Another first principle is, Our own personal identity and continued existence, as far back as we remember anything distinctly.
This we know immediately, and not by reasoning. It seems, indeed, to be a part of the testimony of memory. Everything we remember has such a relation to ourselves as to imply necessarily our existence at the time remembered.
5. Another first principle is, That those things do really exist which we distinctly perceive by our senses, and are what we perceive them to be.
It is too evident to need proof, that all men are by nature led to give implicit faith to the distinct testimony of their senses, long before they are capable of any bias from prejudices of education or of philosophy.
6. Another first principle, I think, is, That we have some degree of power over our actions, and the determinations of our will.
All power must be derived from the fountain of power, and of every good gift. Upon His good pleasure its continuance depends, and it is always subject to His control.
Beings to whom God has given any degree of power, and understanding to direct them to the proper use of it, must be accountable to their Maker. But those who are intrusted with no power can have no account to make; for all good conduct consists in the right use of power; all bad conduct in the abuse of it.
7. Another first principle is—That the natural faculties, by which we distinguish truth from error, are not fallacious. If any man should demand a proof of this, it is impossible to satisfy him. For, suppose it should be mathematically demonstrated, this would signify nothing in this case; because, to judge of a demonstration, a man must trust his faculties, and take for granted the very thing in question.
8. Another first principle relating to existence, is, That there is life and intelligence in our fellow-men with whom we converse.
9. Another first principle I take to be, that certain features of the countenance, sounds of the voice, and gestures of the body, indicate certain thoughts and dispositions of mind.
10. Another first principle appears to me to be—That there is a certain regard due to human testimony in matters of fact, and even to human authority in matters of opinion.
11. There are many events depending upon the will of man, in which there is a self-evident probability, greater or less, according to circumstances.
12. The last principle of contingent truths I mention is, That, in the phænomena of nature, what is to be will probably be like to what has been in similar circumstances.
We must have this conviction as soon as we are capable of learning anything from experience; for all experience is grounded upon a belief that the future will be like the past. Take away this principle, and the experience of an hundred years makes us no wiser with regard to what is to come.
This is one of those principles which, when we grow up and observe the course of nature, we can confirm by reasoning. We perceive that Nature is governed by fixed laws, and that, if it were not so, there could be no such thing as prudence in human conduct; there would be no fitness in any means to promote an end; and what, on one occasion, promoted it, might as probably, on another occasion, obstruct it.
But the principle is necessary for us before we are able to discover it by reasoning, and therefore is made a part of our constitution, and produces its effects before the use of reason.
I do not at all affirm, that those I have mentioned are all the first principles from which we may reason concerning contingent truths. Such enumerations, even when made after much reflection, are seldom perfect.1
[1 ]Ibid., pp. 441-452. (In reprinting this and the following section some passages have been silently omitted.)