Indirect realism is broadly equivalent to the accepted view of perception in natural science that states that we do not and cannot perceive the external world as it really is but know only our ideas and interpretations of the way the world is. The main alternative to representationalism is anti-representationalism, the view according to which perception is not a process of constructing internal representations.
Aristotle was the first to provide a description of direct realism. In On the Soul he describes how a see-er is informed of the object itself by way of the hylomorphic form carried over the intervening material continuum with which the eye is impressed. In medieval philosophy, direct realism was defended by Thomas Aquinas. For example, one perceives an object as spherical precisely because of the way the atoms of the sphere are arranged.
Secondary qualities include colour, smell, and taste. Thomas Reid, a notable member of the Scottish common sense realism was proponent of direct realism. Direct realist views have been attributed to Baruch Spinoza. Hegel followed Kant in adopting empirical realism.
It is the happening of the disclosing of beings, but what about the first horn? Even if reality in itself were law, the core physical properties of objects in space are grounded in the contents of our experience of them.
Wilfrid Sellars in his seminal essay “Empiricism and the Philosophy of Mind”. Illusion creates a problem for naïve realists as it suggests our senses are fallible, perceiving things that aren’t there. In this illusion, the lines are horizontal, despite how they appear.