The theory that the external world is fundamentally immaterial and a dimension of the mind. That is, reality is of the nature of mind or consciousness (non-material). There are many types of idealism, but objective (usually pantheistic) idealism and subjective (usually many minds, one Supreme Mind) idealism are the two major categories. Objects in the world exist but they lack substance. That is, any system that explains the universe in terms of ideas and maintains that all phenomena are subjective. The doctrine maintains that material objects are copies of eternal models. Summed up in the mantra "Reality is mental" by the Greek pre-Socratic philosopher Parmenides (c. 510–450 BC).