In common usage, existence is the world we are aware of through our senses, and that persists independently without them. In academic philosophy the word has a more specialized meaning, being contrasted with essence, which specifies different forms of existence as well as different identity conditions for objects and properties. Philosophers investigate questions such as ""What exists?"" ""How do we know?"" ""To what extent are the senses a reliable guide to existence?"" ""What is the meaning, if any, of assertions of the existence of categories, ideas, and abstractions?"" Ontology is the philosophical study of the nature of being, existence or reality in general, as well as of the basic categories of being and their relations.