Full Idea
The idea of body and body, that is, mind and body, are one and the same individual conceived now under the attribute of thought, now under the attribute of extension.
Clarification
'Extension' means occupying physical space
Gist of Idea
Mind and body are the same thing, sometimes seen as thought, and sometimes as extension
Source
Baruch de Spinoza (The Ethics [1675], II Pr 21)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.102
A Reaction
This is an appealingly modern view, but a bit glib. Phenomenologically, the mind seen as thought and the body seen as extension are about as wildly different as it is possible to be. This needs explanation.