By transcendental idealism I mean the doctrine that appearances are to be regarded as being, one and all, representations only, not things in themselves, and that time and space are therefore only sensible forms of our intuition, not determinations given as existing by themselves, nor condition of objecs viewed as things in themselves. To this idealism there is opposed a transcendental realism which regards time and space as something given in themselves, independently of our sensibility. The transcendental realist thus interprets outer appearances (their reality taken as granted) as things in themselves, which exist independently of us and our sensibility, and which are therefore outside us – the phrase 'outside us' being interpreted in conformity with pure concepts of understanding. It is in fact, this transcendental realist who afterwards plays the part of empirical idealist. After wrongly supposing that objects of the senses, if they are to be external, must have an existence by themselves, and independently of the senses, he finds that, judged from this point of view, all our sensuous representations are inadequate to establish their reality.
From the Critique of Pure Reason (Everyman p 296) - by Immanuel Kant