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 P: 31 So I want to start learning about topological spaces, however I couldn't even get past the definition. My book states: DEFINITION (Open sets, neighborhoods) Let E be an arbitrary set. A topology on E is the data of a set O of subsets of E, called the open subsets of E (for the given topology), which satisfy the following three properties: i) $$\emptyset \in O$$ and $$E \in O$$ I won't write the other two, but the thing I do not understand is how on earth can O contain open sets? I checked wikipedia and they give basically the same definition as my book, but they have examples, one example is that X={1, 2, 3, 4} with the collection τ={{}, {1, 2, 3, 4}} form a topology. So I guess τ is like the O in my books definition... but again, how is it possible that {1, 2, 3, 4} is an open set? I thought that an open set requires there to be an $\epsilon>0$ that can be added to any element in the open set such that the result is still in that open set. However... I can think of many epsilons that can be added to elements in {1, 2, 3, 4} that do not give elements of the set, so why is it considered an open set? Basically my question is... how is it possible for a finite set of natural numbers to be an open set?