A subject is an observer and an object is a thing observed. This concept is especially important in Continental philosophy, where 'the subject' is a central term in debates over the nature of the self. The nature of the subject is also central in debates over the nature of subjective experience within the Anglo-American tradition of analytical philosophy.
The sharp distinction between subject and object corresponds to the distinction, in the philosophy of René Descartes, between thought and extension. Descartes believed that thought (subjectivity) was the essence of the mind, and that extension (the occupation of space) was the essence of matter.