Spinoza conceives infinity as all-inclusiveness with respect to a certain kind (genere). Thus, a limited extended region is finite by virtue of other ex? tended regions not included in it, but extension as such is infinite because there is nothing extended outside it. Moreover, according to him, infinity does not mean the containing of many items or being divisible ad infini tum. We should think of it as the organizing principle of an all-compre? hending system, where the principle is identified with the system itself. In modern terminology one could say that Spinoza characterizes infinity not in terms of cardinality (i.e., numerical quantity), but as a closure prop? erty the exhaustion of its kind (suo genere). Although today "infinite" brings to mind, iirst and foremost, a numerical concept, Spinoza's usage fits well certain fundamental intuitions. Certain philosophical puzzles that have come to play a central role result from attempts to conceive all ex? haustive closed systems, or to "close up" given systems in certain direc? tions. The "closing up" tendency appears to be a basic element of human thought and, in particular, it is manifested in mathematics, where thought is, in a sense, given free reign. I shall discuss paradoxes of two categories: Those that (for want of a better name) I shall ref?r to as logical paradoxes include the classical cases of set-theoretic and semantic paradoxes (Russell, Burali-Forti, Berry, the Liar etc). The others are what I shall call epistemic Gedankenexperiments; in these a rational judgement, or decision, is demanded of someone under certain hypothetical, but presumably concrete, conditions. An example of this kind is Newcomb's paradox, another is a puzzle suggested to me by Gideon Schwarz (both will be analyzed later, but for reasons of length the analysis of Newcomb's paradox is postponed to part II). The obvious differences between the two kinds notwithstanding, they share the same
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