This book offers a complete overview of the complex topic of reasonableness. Ranging through ethics, political theory and legal theory, the author shows how 'reasonable' is an unescapable evaluative term. One uses this concept to evaluate positively or negatively - -it is unreasonable!' - both our conduct and that of others. However, we are often not aware of the presuppositions behind our use of the concept. Especially the legal use, both in civil and in common law, reveals a large variety of meanings and the so called 'multifacetedness' of reasonableness. In an age in which the environmental crisis or 'global climate change' threatens our style of life we badly need criteria of evaluation that we all can share. This book therefore provides a unitary conception that connects to the ethics of virtues as the fabric that links the many uses of 'the reasonable'.