We prove a characterization theorem for filters, proper filters and ultrafilters which is a kind of converse of Łoś's theorem. It is more natural than the usual intuition of these terms as large sets of coordinates, which is actually unconvincing in the case of ultrafilters. As a bonus, we get a very simple proof of Łoś's theorem.
 J. M. Łoś, Quelques Remarques, Théorèmes Et Problèmes Sur Les Classes Définissables D'algèbres, Studies in Logic and the Foundations of Mathematics, vol. 16 (1955), Mathematical Interpretation of Formal Systems, pp. 98–113.
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