This book, first published in 2000, is about the nature of skeptical arguments and their role in philosophical inquiry. John Greco delineates three main theses: that a number of historically prominent skeptical arguments make no obvious mistake, and therefore cannot be easily dismissed; that the analysis of skeptical arguments is philosophically useful and important, and should therefore have a central place in the methodology of philosophy; and that taking skeptical arguments seriously requires us to adopt an externalist, reliabilist epistemology. Greco argues that the importance of skeptical arguments is methodological. It is further argued that taking skeptical arguments seriously requires us to adopt a version of 'virtue epistemology', or a theory of knowledge that makes intellectual virtue central in the analysis of knowledge. The above methodology has consequences for moral and religious epistemology; in particular, a theory of moral perception is defended.