The Casual Theory of Perception

John Locke

The first part of this excerpt from Essay Concerning Human Understanding sets out Locke's distinction between ideas and objects themselves and his distinction between primary and secondary qualities.

Understanding these distinctions and how they relate to each other lays the groundwork for Locke's thesis about what one can know.

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Is Your Red The Same as My Red?https://www.youtube.com/watch?v=evQsOFQju08

Idea

❖ First, on Locke's account an idea is whatever is "the immediate object of perception, thought, or understanding.”

❖ An idea can be produced by objects by virtue of those objects' qualities

Primary Qualities❖ For Locke, primary qualities are those properties of an object that are

not related by definition to perceivers.

❖ The primary qualities are size, shape, motion, number, and solidity.

❖ We might say that the object has these properties ‘in and of itself’.

❖ Primary qualities, Locke says, are ‘inseparable’ from a physical object, whatever changes it goes through.

❖ For example, physical objects always have some shape and size.

❖ These properties don’t depend, either conceptually or for their existence, on whether and how the object is perceived.

Secondary Qualities❖ Secondary qualities are related to perceivers by definition.

❖ As we saw, color, by definition, is something that is experienced in vision.

❖ So it is a property that an object can have only in relation to its being seen by someone.

❖ The other secondary qualities are temperature, smell, taste, and sound.

❖ Secondary qualities aren’t possessed by all physical objects, e.g. plain glass doesn’t have a color or a smell.

❖ And they aren’t even possessed by the same physical object at different times, e.g. glass is made from sand, and sand does have color.

❖ So sand loses its color completely when it is made into glass.

Do secondary properties exist ‘in the object’ or ‘in the mind’ of the perceiver?

Primary and Secondary Qualities❖ In his discussion of primary and secondary qualities, Locke

claims that the ‘ideas’ (the sense impressions) of primary qualities – our sense-data of shape, size, motion and so on – ‘resemble’ the primary qualities that the object we are perceiving has.

❖ However, the sense impressions of secondary qualities – our sense-data of color, smell and so on – don’t resemble the object at all.

❖ The experience of seeing red, for example, just isn’t like detecting a vibration – yet it is.

Does this show that secondary qualities exist in the mind?

Locke’s ‘official’ theory is that secondary qualities are properties of the object that are related to its being

perceived by us.

Secondary Qualities…❖ are nothing in the objects themselves but powers to produce various

sensations in us.

❖ Here Locke identifies the secondary quality with the power of the object.

❖ He goes on to argue that this power should be understood in terms of primary properties of the object’s ‘most minute parts’, or as we would now put it, in terms of its atomic and molecular structure.

❖ As we suggested, secondary qualities can be causally explained in terms of primary properties.

Consider the act of viewing a snowball

❖ On Locke's view we have ideas of (i.e., immediately perceive) roundness and whiteness that are caused by some quality (or power) in the snowball to produce those sensations.

❖ Those qualities in the snowball are of two sorts.

❖ There are primary qualities that Locke says are in the objects themselves; these include solidity, extension, motion, and number. Objects have primary qualities regardless of circumstances, and regardless of whether those qualities are perceived.

❖ In addition, there are secondary qualities, which are the powers to produce certain ideas by the operation of an object's primary qualities upon a perceiving subject (a person).

❖ Thus, colors, sounds, and tastes are the powers to produce certain ideas in a perceiving subject by the operation of an object's bulk, extension, number, etc. on that subject.

What can we know?

❖ Locke claims that we can know of our own existence by intuition, and that we can know of God's existence by reason (though there is no argument for this in the excerpt), but that the "knowledge of the existence of any other thing we can have only by sensation.

❖ When one has an idea and that idea is presently being caused by the operation of an object's primary qualities on one's perceptive faculties, one knows that there is an object and that it has certain qualities.

External Objects❖ Locke argues that the sense perceptions we get from external objects are

reliable producers of knowledge of those objects for several reasons.

❖ First, it is clear that sense organs themselves do not produce sensation alone; otherwise one would see color in the dark.

❖ Second, where external objects cause sensation, one cannot avoid the idea as one can with a memory or imagined sensation.

❖ Third, the pleasure or pain that accompany some sensations are caused by external objects and are not replicated in the memory of those sensations.

❖ Finally, sensations caused by external objects are verified by each other; we can check whether we actually see a fire by simultaneously feeling its heat, for example.

Note…

❖ however, that on Locke's account this is all that one knows.

❖ An idea that one has in virtue of memory does not constitute knowledge of the object that caused the sensation in the first place, for that object may no longer exist.

❖ That is, the connection between past perceptions and present existence is too tenuous to call knowledge.