In the process of examining the relationship between the mind that thinks and indubitably exists, and the perceptions of his senses, Descartes draws a distinction between the properties that are perceived of a thing, and the thing itself. For this exposition, he uses the example of a chunk of beeswax.
"Take, for example, this piece of wax; it is quite fresh, having been but recently taken from the beehive; it has not yet lost the sweetness of the honey it contained; it still retains somewhat of the odor of the flowers from which it was gathered; its color, figure, size, are apparent ( to the sight ); it is hard, cold, easily handled; and sounds when struck upon with the finger."[Meditation II - 11]
Then he places the wax next to the fire and observes how the lump of wax appears to the senses to change with the heat.
"[W]hat remained of the taste exhales, the smell evaporates, the color changes, its figure is destroyed, its size increases, it becomes liquid, it grows hot, it can hardly be handled, and, although struck upon, it emits no sound."[Meditation II - 11]
In short, Descartes observes that none of the perceptible properties of the wax before the application of heat are the same as the perceptible properties of the wax after. Yet it remains indisputably the same piece of wax. So the wax itself must be none of its perceptible properties. But the only way that we perceive the wax with our senses, is by perceiving its perceptible properties - and we have just seen that those properties need not remain the same over time.
Hence, in order to properly grasp the nature of the lump of wax as a single thing rather than as a changing collection of properties, we cannot be using our senses. We must be using our minds. Descartes concludes "And so something which I thought I was seeing with my eyes is in fact grasped solely by the faculty of judgment which is in my mind."Hence, his conclusion that the nature of the piece of wax as wax, as distinguished from any of the particular sensible properties of the wax at some particular instant, "is ... perceived by the mind alone". [Meditation II -- 12]
Descartes little experiment with the wax serves two purposes. Firstly, Descartes uses it to highlight the fact that we cannot rely on our senses as the basis of knowledge because the senses deliver contradictory information on the nature of the wax before and after melting near the fire. And secondly, Descartes uses it to argue that it is the power of the mind alone that can correctly "perceive"the nature of the wax. Descartes is emphasizing the power of the mind to perceive (and here he is using "perceive"to mean "reflect on"or "attend to"or "understand") its own ideas. Understanding comes from pure reason, as distinct from the power of sensing (e.g., seeing or touching) and the power of imagination.
It is significant that Descartes, in his Meditations, uses the word "perceive"in two different, and somewhat conflicting, senses. Firstly, he uses "perceive"to describe what the senses do when they detect the properties of the wax -- when they examine the external world. And secondly, he uses "perceive"to describe what the mind does when it reaches a judgement about the nature of that wax -- when it examines its own ideas. By failing to keep this distinction clear, he often confuses his audience. When he talks about a "clear and distinct perception"of the wax, he is employing his second sense of perception (i.e. "understanding") to describe the idea that his mind develops of the true nature of the wax as something separate from its perceptible properties. He is not talking about anything that his senses report on the properties of the wax.
What leads Descartes to this position is his belief that the Mind is fundamentally different from the Body -- his Mind-Body dualism. Sensory perception is a function of the body, because the sensory organs are indisputably part of the body. The Mind, being separate from the Body must somehow be "presented"with the reports of the senses. Although he does not actually describe it this way, Descartes imagines that there is a "Cartesian Theatre"somewhere betwixt the Body and the Mind wherein the Mind "views"the reports of the senses. Since in Descartes'way of thinking it is the Mind that contains the functions of temporal integration, it must be the Mind that integrates the changing sensory reports of the properties of the was, and develops the integrated "clear and distinct perception"of the lump of wax as something beyond the changing collections of properties.
This separation of Mind and Body is the fundamental keystone of Descartes'philosophy. It leads him to conclude that the Mind has a privileged access to, and hence a better understanding of, its own contents, than it does of the observed properties of the external world. Hence, when Descartes goes about applying his "methodological doubt"(Meditation I) in his search for an indubitable basis for knowledge, he permits his "evil demon"to fool him about all things external to his Mind. But Descartes does not permit that foolery to penetrate across the Mind-Body barrier into the operations of his Mind, and the "clear and distinct perceptions"(meaning "understandings") that his Mind can develop of its own ideas.
In Meditation III, Descartes provides a "proof"for the existence of God that depends only upon logically necessary truths. In other words, on ideas that are independent of the reports of his sensory perceptions. This proof would be impossible if Descartes permitted his "evil demon"to fool him about which of his ideas are logically necessary. And without this proof, Descartes'entire access to the real world disintegrates. So the separation of the Body's sensory perception of properties from the Mind's "perception"of its own ideas is a fundamentally necessary component of Descartes'philosophy.
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