This book is about the nature of dispositional properties, or dispositions. It is hard to give an uncontroversial definition of the notion of a disposition, since its very definition is one of the matters under dispute. But we can make a start with the following preliminary definition: a disposition is a property (such as solubility, fragility, elasticity) whose instantiation entails that the thing which has the property would change, or bring about some change, under certain conditions. For instance, to say that some object is soluble is to say that it would dissolve if put in water; to say that something is fragile is to say that it would break if (for instance) dropped in suitable circumstances; to say that something is elastic is to say that it would stretch when pulled. The fragility (solubility, elasticity) is a disposition; the breaking (dissolving, stretching) is the manifestation of the disposition.