Descartes is justly considered the father of modernphilosophy, primarily and generally because hetaught reason to stand on its own feet byinstructing people to use their own minds, which hadrested until then on the Bible on the one hand andAristotle on the other. He is the father in aparticular and more narrow sense because he was thefirst to become aware of the problem around whichall philosophizing has mainly revolved since then:the problem of the ideal and the real, i.e. thequestion what in our cognition is objective and whatsubjective, thus what is to be ascribed to anythings distinct from ourselves and what toourselves. – For in our heads images arise,occasioned not internally – originating from choiceor from the association of ideas – but externally.These images alone are what is immediately known tous, what is given. What kind of relation might theyhave to things that exist completely separately fromand independently of us and that would somehow causethese images? Are we certain that such things areeven there? And, in case they are, do the imagesgive us any information about their constitution? –This is the problem, and consequently, for the lasttwo hundred years, it has been the main endeavour ofthe philosophers to separate the ideal, that is,that which belongs exclusively to our cognition,from the real, that is, that which existsindependently of our cognition, clearly in awell-executed, clean cut, and thus to determinetheir relation to one another.

In reality, neither the ancient nor the scholasticphilosophers seem to have become distinctly aware ofthis primordial philosophical problem, although wefind a trace of it, as idealism and even as thedoctrine of the ideality of time, in Plotinus, in Ennead III,Book 7, ch. 10, where he teaches that the soul madethe world by emerging from eternity into time. Itsays there, for example: ‘for there is no otherplace for the universe than soul’ and also: ‘Time,however, is not to be conceived as outside of soul,just as eternity is not outside being’; which infact already pronounces Kant's ideality of time.