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What makes sets, or more precisely, the category {\bf Set} important in Mathematics are the well known {\it two} specific ways in which arbitrary mappings $f : X \longrightarrow Y$ between any two sets $X, Y$ can {\it fail} to be bijections. Namely, they can fail to be injective, and/or to be surjective. As for bijective mappings they are rather trivial, since with some relabeling of their domains or ranges, they simply become permutations, or even identity mappings. \\ To the above, one may add the {\it third} property of sets, namely that, between any two nonvoid sets there exist mappings. \\ These three properties turn out to be at the root of much of the interest which the category {\bf Set} has in Mathematics. Specifically, these properties create a certain {\it dynamics}, or for that matter, lack of it, on the level of the category {\bf Set} and of some of its subcategories. |
