A great deal of our knowledge of the world is gained via perception (i.e. via our senses). What is problematic about perceptual experience is brought out via the argument from illusion. In essence, this states that since a situation in which we are deceived about the world could be one in which we have, it seems, exactly the same experiences as we would have in a corresponding undeceived case, we don’t directly experience the world at all. The conception of perceptual knowledge suggested by the argument from illusion is that of indirect realism. This holds that there is an objective world out there, one that is independent of our experience of it, but that we can only know this world indirectly through experience. Indirect realism can account for the distinction between those primary properties or qualities of an object that are inherent in the object, such as its shape, and those secondary properties or qualities of an object that are dependent upon the perceiver, such as its colour. An alternative to indirect realism is idealism. This maintains that the world is constructed out of appearances and does not extend beyond it – that is, there is no mind-independent world. A variant of idealism is known as transcendental idealism. This maintains that while we are unable to have any experiential knowledge of the external world, nevertheless we can use reason to show that there must be an external world that gives rise to our experiences. There is also a common-sense view of perceptual experience called direct realism. This holds that we can directly experience the world, and so rejects the conclusion usually derived from the argument from illusion that direct experience of the world is impossible.