Number is not abstracted from things in the way color, weight, and hardness are, nor is it a property of things in the external world: it belongs to a concept. The statement that a course of lectures has been given every Monday and Wednesday for three terms contains a numerical statement about a concept, not about a day or a week. If I say 'Venus has zero moons', there simply is no moon or agglomeration of moons for me to attach the zero to.
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