This chapter discusses the problem involved in the origin of species. It may be that logic is wrong, and that it would be better to leave it undefined, accepting the fact that all biologists have a pragmatic idea at the back of their heads. It may even be that the word is undefinable. There was a division in the way the Synthesis Darwinians approached species, which can be traced back to Darwin's own published ambiguity on the subject. The chapter describes the modern debate into context, especially claims of conceptual novelty. The application of a biological species definition is possible only in well-studied taxonomic groups, since it is based on a rather exact knowledge of geographical distribution and on the certainty of the absence of interbreeding with other similar species. Ernst Mayr’s conception of species is one of dispositions to behave in various ways.