Modern philosophy has taught us, in certain cases, to controvert the position, that the causes of our sensations are like to the sensations themselves. Locke in particular has called the attention of the reasoning part of mankind to the consideration, that heat and cold, sweet and bitter, and odour offensive or otherwise, are perceptions, which imply a percipient being, and cannot exist in inanimate substances. We might with equal propriety ascribe pain to the whip that beats us, or pleasure to the slight alternation of contact in the person or thing that tickles us, as suppose that heat and cold, or taste, or smell are any thing but sensations.
The same philosophers who have called our attention to these remarks, have proceeded to shew that the causes of our sensations of sound and colour have no precise correspondence, do not tally with the sensations we receive. Sound is the result of a percussion of the air. Colour is produced by the reflection of the rays of light; so that the same object, placed in a position, different as to the spectator, but in itself remaining unaltered, will produce in him a sensation of different colours, or shades of colour, now blue, now green, now brown, now black, and so on.
This is the doctrine of Newton, as well as of Locke.
It follows that, if there were no percipient being to receive these sensations, there would be no heat or cold, no taste, no smell, no sound, and no colour.
Aware of this difference between our sensations in certain cases and the causes of these sensations, Locke has divided the qualities of substances in the material universe into primary and secondary, the sensations we receive of the primary representing the actual qualities of material substances, but the sensations we receive of what he calls the secondary having no proper resemblance to the causes that produce them.
Now, if we proceed in the spirit of severe analysis to examine the primary qualities of matter, we shall not perhaps find so marked a distinction between those and the secondary, as the statement of Locke would have led us to imagine.
The Optics of Newton were published fourteen years later than Locke's Essay concerning Human Understanding.
In endeavouring to account for the uninterrupted transmission of rays of light through transparent substances, however hard they may be found to be, Newton has these observations.
"Bodies are much more rare and porous, than is commonly believed.
Water is nineteen times lighter, and by consequence nineteen times rarer, than gold; and gold is so rare, as very readily, and without the least opposition, to transmit the magnetic effluvia, and easily to admit quicksilver into its pores, and to let water pass through it. From all which we may conclude, that gold has more pores than solid parts, and by consequence that water has above forty times more pores than parts. And he that shall find out an hypothesis, by which water may be so rare, and yet not capable of compression by force, may doubtless, by the same hypothesis, make gold, and water, and all other bodies, as much rarer as he pleases, so that light may find a ready passage through transparent substances[43]."
[43] Newton, Optics, Book II, Part III, Prop. viii.
Again: "The colours of bodies arise from the magnitude of the particles that reflect them. Now, if we conceive these particles of bodies to be so disposed among themselves, that the intervals, or empty spaces between them, may be equal in magnitude to them all; and that these particles may be composed of other particles much smaller, which have as much empty space between them as equals all the magnitudes of these smaller particles; and that in like manner these smaller particles are again composed of others much smaller, all which together are equal to all the pores, or empty spaces, between them; and so on perpetually till you come to solid particles, such as have no pores, or empty spaces within them: and if in any gross body there be, for instance, three such degrees of particles, the least of which are solid; this body will have seven times more pores than solid parts. But if there be four such degrees of particles, the least of which are solid, the body will have fifteen times more pores than solid parts. If there be five degrees, the body will have one and thirty times more pores than solid parts. If six degrees, the body will have sixty and three times more pores than solid parts.
And so on perpetually[44]."
[44] Ibid.
In the Queries annexed to the Optics, Newton further suggests an opinion, that the rays of light are repelled by bodies without immediate contact. He observes that:
"Where attraction ceases, there a repulsive virtue ought to succeed. And that there is such a virtue, seems to follow from the reflexions and inflexions of the rays of light. For the rays are repelled by bodies, in both these cases, without the immediate contact of the reflecting or inflecting body. It seems also to follow from the emission of light; the ray, so soon as it is shaken off from a shining body by the vibrating motion of the parts of the body, and gets beyond the reach of attraction, being driven away with exceeding great velocity. For that force, which is sufficient to turn it back in reflexion, may be sufficient to emit it. It seems also to follow from the production of air and vapour: the particles, when they are shaken off from bodies by heat or fermentation, so soon as they are beyond the reach of the attraction of the body, receding from it and also from one another, with great strength; and keeping at a distance, so as sometimes to take up a million of times more space than they did before, in the form of a dense body."