Following Maimonides, Spinoza defined substance as "that which is in itself and is conceived through itself", meaning that it can be understood without any reference to anything external. Being conceptually independent also means that the same thing is ontologically independent, depending on nothing else for its existence and being the 'cause of itself' ("causa sui"). A mode is something which cannot exist independently but rather must do so as part of something else on which it depends, including properties (for example colour), relations (such as size) and individual things. Modes can be further divided into 'finite' and 'infinite' ones, with the latter being evident in every finite mode (he gives the examples of "motion" and "rest"). The traditional understanding of an attribute in philosophy is similar to Spinoza's modes, though he uses that word differently. To him, an attribute is "that which the intellect perceives as constituting the essence of substance", and there are possibly an infinite number of them. It is the essential nature which is "attributed" to reality by intellect.