ATOCHA ALISEDA LOGICS IN SCIENTIFIC DISCOVERY ABSTRACT. In this paper I argue for a place for logic in scientific methodology, at the same level as that of computational and historical approaches. While it is well known that a a whole generation of philosophers dismissed Logical Positivism (not just for the logic though), there are at least two reasons to recon- sider logical approaches in the philosophy of science. On the one hand, the present situation in logical research has gone far beyond the formal developments that deductive logic reached last century, and new research includes the formaliza- tion of several other types of reasoning, like induction and abduction. On the other hand, we call for a balanced Philosophy of Science, one in which both methods, the formal and the historical may be complementary, together providing a pluralistic view of science, in which no method is the predominant one. KEY WORDS: abduction, heuristics, logic, scientific discovery 1. INTRODUCTION A central theme in the study of human reasoning is the construction of explanations that give us an understanding of the world we live in. Broadly speaking, abduction is a reasoning process invoked to explain a puzzling observation. If we wake up, and the lawn is wet, we might explain this observation by assuming that it must have rained, or by assuming that the sprinklers have been on. This is a practical setting found in our day-to-day common sense reasoning. Abduction also occurs in more theoretical scientific contexts. For instance, it has been claimed that Johannes Kepler’s great discovery that the orbit of the planets is elliptical rather than circular was a prime piece of abductive reasoning (Hanson, 1961; Peirce 2.623, 1958). What initially led to this discovery was his observation that the longitudes of Mars did not fit circular orbits. However, before even dreaming that the best explanation involved ellipses instead of circles, he tried several other forms. Moreover, Kepler had to make several other assumptions about the planetary system, without which his discovery does not work. His heliocentric view allowed Foundation of Science 9: 339–363, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.