Fundamenta Informaticae 142 (2015) 269–284 269 DOI 10.3233/FI-2015-1294 IOS Press Where Rough Sets and Fuzzy Sets Meet Lech Polkowski ∗ Polish–Japanese Institute of Information Technology Koszykowa 86, 02008 Warsaw, Poland and Department of Mathematics and Computer Science University of Warmia and Mazury Sloneczna str. 54, 10560 Olsztyn, Poland Maria Semeniuk–Polkowska Chair of Formal Linguistics, Warsaw University Browarna 8/12, 00650 Warsaw,Poland polkow@pjwstk.edu.pl; m.polkowska@uw.edu.pl Oh, Rough is rough and Fuzzy is fuzzy but sometimes the twain shall meet [Rudyard Kipling. The Ballad of East and West. 1st line paraphrase] To Lotfi Asker Zadeh To the memory of Zdzislaw Pawlak Abstract. The title of this note announces an attempt at pointing to particular points where rough set and fuzzy set approaches to decision making are close to each other, the closeness meaning that formal approaches to behaviour of the two at those points can be given in an analogous form. This by no means implies that the two can be unified as theories dealing with uncertainty. As the notion of truth for rough set decision rules is well established, we propose a notion of truth for fuzzy decision rules and we seek an analogy between the two. In order to introduce an analogous form of graded notion of truth for decision rules in both theories, we introduce a new context in which to set this notion. This context is based on our earlier results concerning rough mereological granular logics and their relevance for rough decision rules. To make our exposition satisfactorily complete, we recall our approach to granularity based on rough mereology. ∗ Address for correspondence: Polish–Japanese Institute of Information Technology, Koszykowa 86, 02008 Warsaw, Poland