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• Selections From the Scottish Philosophy of Common Sense
• Introduction
• Thomas Reid
• I.—: Introduction to the Philosophy of Common Sense
• § 1.: The Importance of the Subject, and the Means of Prosecuting It
• § 2.: The Impediments to Our Knowledge of the Mind
• II.—: Analysis of a Typical Sensation
• § 1.: The Sensation Considered Abstractly
• § 2.: Sensation and Remembrance, Natural Principles of Belief
• § 3.: Judgment and Belief In Some Cases Precede Simple Apprehension
• § 4.: Two Theories of the Nature of Belief Refuted—conclusions From What Hath Been Said
• § 5.: Apology For Metaphysical Absurdities—sensation Without a Sentient, a Consequence of the Theory of Ideas—consequences of This Strange Opinion
• § 6.: The Conception and Belief of a Sentient Being Or Mind Is Suggested By Our Constitution—the Notion of Relations Not Always Got By Comparing the Related Ideas
• § 7.: There Is a Quality Or Virtue In Bodies, Which We Call Their Smell—how This Is Connected In the Imagination With the Sensation
• § 8.: That There Is a Principle In Human Nature, From Which the Notion of This, As Well As All Other Natural Virtues Or Causes, Is Derived
• § 9.: Whether In Sensation the Mind Is Active Or Passive?
• III.—: Knowledge and Reality
• § 1.: Of Hardness
• § 2.: Of Natural Signs
• § 3.: Of Extension
• § 4.: Of the Visible Appearances of Objects
• § 5.: Of Perception In General
• § 6.: Of the Process of Nature In Perception
• Appendix: of Cause and Power
• IV.—: The Operations of the Mind
• § 1.: Principles Taken For Granted
• § 2.: Of Hypotheses and Analogy
• § 3.: Of Perception
• § 4.: Of Sensation
• § 5.: Of Primary and Secondary Qualities
• § 6.: Of Conception
• § 7.: Of Judgment
• § 8.: Of Common Sense
• § 9.: The First Principles of Contingent Truths
• § 10.: First Principles of Necessary Truths
• V.—: Of Morals
• § 1.: Of Benevolent Affection In General
• § 2.: There Are Rational Principles of Action In Man
• § 3.: Of Regard to Our Good On the Whole
• § 4.: Of the Notion of Duty, Rectitude, Moral Obligation
• § 5.: Observations Concerning Conscience
• § 6.: That Moral Approbation Implies a Real Judgment
• Adam Ferguson
• Of Man’s Progressive Nature
• James Beattie
• Of the Perception of Truth In General
• Dugald Stewart
• I.—: Of the Object of Philosophy, and the Method of Prosecuting Philosophical Inquiries
• II.—: Of the Association of Ideas
• III.—: Of the Power Which the Mind Has Over the Train of Its Thoughts
• IV.—: Of the Influence of Association On Our Active Principles, and On Our Moral Judgments
• V.—: Of Certain Laws of Belief, Inseparably Connected With the Exercise of Consciousness, Memory, Perception, and Reasoning

§ 5.

Of Primary and Secondary Qualities

Every one knows that extension, divisibility, figure, motion, solidity, hardness, softness, and fluidity, were by Mr Locke called primary qualities of body; and that sound, colour, taste, smell, and heat or cold, were called secondary qualities. Is there a just foundation for this distinction? Is there anything common to the primary which belongs not to the secondary? And what is it?

I answer, That there appears to me to be a real foundation for the distinction; and it is this—that our senses give us a direct and a distinct notion of the primary qualities, and inform us what they are in themselves. But of the secondary qualities, our senses give us only a relative and obscure notion. They inform us only, that they are qualities that affect us in a certain manner—that is, produce in us a certain sensation; but as to what they are in themselves, our senses leave us in the dark.

Every man capable of reflection may easily satisfy himself that he has a perfectly clear and distinct notion of extension, divisibility, figure, and motion. The solidity of a body means no more but that it excludes other bodies from occupying the same place at the same time. Hardness, softness, and fluidity are different degrees of cohesion in the parts of a body. It is fluid when it has no sensible cohesion; soft, when the cohesion is weak; and hard, when it is strong. Of the cause of this cohesion we are ignorant, but the thing itself we understand perfectly, being immediately informed of it by the sense of touch. It is evident, therefore, that of the primary qualities we have a clear and distinct notion; we know what they are, though we may be ignorant of their causes.

I observed, farther, that the notion we have of primary qualities is direct, and not relative only. A relative notion of a thing, is, strictly speaking, no notion of the thing at all, but only of some relation which it bears to something else.

Thus, gravity sometimes signifies the tendency of bodies towards the earth; sometimes it signifies the cause of that tendency. When it means the first, I have a direct and distinct notion of gravity; I see it, and feel it, and know perfectly what it is; but this tendency must have a cause. We give the same name to the cause; and that cause has been an object of thought and of speculation. Now, what notion have we of this cause when we think and reason about it? It is evident we think of it as an unknown cause, of a known effect. This is a relative notion; and it must be obscure, because it gives us no conception of what the thing is, but of what relation it bears to something else. Every relation which a thing unknown bears to something that is known, may give a relative notion of it; and there are many objects of thought and of discourse of which our faculties can give no better than a relative notion.

Having premised these things to explain what is meant by a relative notion, it is evident that our notion of primary qualities is not of this kind; we know what they are, and not barely what relation they bear to something else.

It is otherwise with secondary qualities. If you ask me, what is that quality or modification in a rose which I call its smell, I am at a loss to answer directly. Upon reflection, I find, that I have a distinct notion of the sensation which it produces in my mind. But there can be nothing like to this sensation in the rose, because it is insentient. The quality in the rose is something which occasions the sensation in me; but what that something is, I know not. My senses give me no information upon this point. The only notion, therefore, my senses give is this—that smell in the rose is an unknown quality or modification, which is the cause or occasion of a sensation which I know well. The relation which this unknown quality bears to the sensation with which nature hath connected it, is all I learn from the sense of smelling; but this is evidently a relative notion. The same reasoning will apply to every secondary quality.1

[1 ]Ibid., pp. 313-314.