These terms mean literally ‘things that appear’ and ‘things that are thought.’ … [In Kant’s thought, the] intelligible world of noumena is known by pure reason, which gives us knowledge of things as they are.
Things in the sensible world (phenomena) are known through our senses and known only as they appear. To know noumena we must abstract from and exclude sensible concepts such as space and time.
Kant called the determination of noumena and phenomena the ‘noblest enterprise of antiquity’, but in the Critique of Pure Reason he denied that noumena as objects of pure reason are objects of knowledge, since reason gives knowledge only of objects of sensible intuition (phenomena) . Noumena ‘in the negative sense’ are objects of which we have no sensible intuition and hence no knowledge at all; these are things-in-themselves. Noumena ‘in the positive sense’ (e.g. the soul and God) are conceived of as objects of intellectual intuition, a mode of knowledge which man does not possess. In neither sense, therefore, can noumena be known. For both Plato and Kant, nevertheless, conceptions of noumena and the intelligible world are foundational for ethical theory. [The Oxford Companion to Philosophy]