In general, the doctrine according to which our knowledge is confined to the content of our own minds.   

Absolute Idealism.— The philosophical position according to which a single spiritual being is the only thing that exists and, hence, that the whole of reality is identical with this being. All that one knows is, in some manner, contained within this spiritual being. 

Platonic Idealism.—A somewhat misleading expression, referring to the extreme realism of Plato, according to whom human knowledge is a kind of remembrance of the Forms (or “Ideas”) known by the soul prior to the latter’s union with the body.   

Transcendental Idealism.— The philosophical claim that we cannot know things as they are in themselves but only as they are structured by our cognitive powers. This philosophical position is generally associated with the philosophy of Immanuel Kant. According to Kant, our cognitive powers are taken to act upon the inputs from the external world and not merely to receive them passively.