In the question of all the ideas we have about outer reality – for example, the sun and the moon – there is the possibility that they are fantasies. But outer reality also has certain characteristics that we can perceive with our reason. These are the mathematical properties, or, in other words, the kinds of things that are measurable, such as length, breadth, and depth. Such “quantitative” properties are just as clear and distinct to my reason as the fact that I am a thinking being. “Qualitative” properties such as color, smell, and taste, on the other hand, are linked to our sense perception
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