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INSTITUTES OF METAPHYSIC.
PROP. V.————
other notion of them than the notion of their secondary qualities.
Character of the primary qualities.5. The primary qualities are said to be of a different character, and to supply the information and the evidence which are wanted. These are principally extension, figure, and solidity. We are cognisant of these qualities, not as mere sensations in ourselves, like heat and cold, colour and sound, but as they exist and show themselves in external things. Heat and cold, colour and sound—in a word, all our sentient modifications—may be so increased in degree as to become unbearable. But our perceptions of the extension and figure and incompressibility of material objects cannot be thus augmented in intensity. By this circumstance our perceptions are distinguished from our sensations: the latter are susceptible of different degrees of vivacity; some amount of bodily pleasure or pain enters into their composition. Not so in the case of our perceptions. Their degree is always the same; they involve no organic pleasure, or the reverse. It is through our perceptions, and not through our sensations, that we are made acquainted with the primary qualities of matter—that is, with the extension, the figure, and the solidity of external objects. It is further alleged that the terms which indicate the primary qualities are not ambiguous, but have only one signification. But the important circumstance to which psychology