For Locke, idea is the immediate object of perception or whatever the mind perceives in itself. The power to produce any idea in our mind is a quality of the object endowed with that power. For example, a snowball has the power to produce in our minds the ideas of white, cold, and round. It means that a snowball has qualities which generate ideas in our understandings through sensation or perception.  

Locke differentiate between primary and secondary qualities. Primary qualities are those that are inseparable from the body in whatever states they are. These are solidity, extension, figure, motion or rest. For instance, if we divide a grain of wheat, it will retain the same qualities. In other words, each part of the grain has still solidity, extension, figure, motion or rest. Secondary qualities are powers to produce various ideas in us by several combinations of primary qualities of the objects. These are sensible qualities since primary qualities of the object operate on any of our senses and thereby produce in us the different ideas of colors, sounds, smells, tastes etc. We can add to this sort barely powers of the object, of which primary qualities change primary qualities of another body. This change operates as well on our senses. For example, fire has a power to make lead fluid.

The question arises as how primary qualities of objects produce their ideas. If we suppose that our minds are not connected to external objects, then some motion must be continued from these objects, by some parts of our bodies to our brains. For instance, we can perceive by the sight the extension, figure, motion of bodies, meaning that some imperceptible bodies must convey to the brain some impulse which produces these ideas. Similarly, the ideas of secondary qualities are also produced by the operation of insensible particles on our senses. Different motions, bulk and number of such particles affect the several organs of our senses, producing in us different sensations of tastes, smells, colors, etc. The idea of sweet scent of a flower, for example, may be caused by particles that are smaller than the particles of air and water.

From distinction between primary and secondary qualities, we can draw the following observation. The ideas of primary qualities of bodies resemble primary qualities and their patterns that really exist in the bodies themselves. Whether we perceive fire or not, the solidity, extension, figure, motion or rest of the parts of fire are really in them. Contrary to primary qualities, secondary ones have no resemblance of the ideas produced in us. For example, heat of fire does not really exist in the fire. At one distance, it produces in us the sensation of warmth. However, if we approach fire too closely, it may produce in us the idea of pain. Similarly, the idea of light of fire does not resemble its quality. We see the light, when our eyes are open. Closing eyes, we let the eyes not see light. In this sense, if we take away all the sensation, all secondary qualities are reduced to their causes, that is primary qualities like bulk, figure and motion of parts.     

In conclusion, primary qualities of objects are real and original, because they are in the things themselves. They operate on our senses, producing ideas of the bulk, figure, solidity, motion. As for secondary qualities of objects, they are two-fold. First, there are immediately perceivable qualities, immediately operating on our bodies to produce ideas of colors, sounds, smells, tastes, etc. Secondly, there are mediately perceivable qualities, or barely powers, making a change in the bulk, figure, texture and motion of another body and producing ideas of colors, sound, smells, tastes of that body. So, these two sorts of secondary qualities result from the different modifications of primary qualities of the object and they do not have resemblance with the ideas produced in us by these powers.  

Bibliography:

  • Locke, John, Essay Concerning Human Understanding in Modern Philosophy, an anthology of primary sources, second edition, edited by Roger Ariew and Eric Watkins, Hackett Publishing Company, Inc. 2009