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The epistemological question of whether the world we see around us is the real world itself or merely an internal perceptual copy of that world generated by neural processes in our brain.

Direct Realism : Perceptual realism is the common sense view that tables, chairs and cups of coffee exist independently of perceivers. Direct realists also claim that it is with such objects that we directly engage.

Indirect realism is the philosophical position that our conscious experience is not of the real world itself but of an internal representation, a miniature virtual-reality replica of the world. It 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.It denies that 'first hand knowledge' is a coherent concept, since knowledge is always via some means. Our ideas of the world are interpretations of sensory input derived from an external world that is real. The indirect realist agrees that the coffee cup exists independently of me. However, through perception I do not directly engage with this cup; there is a perceptual intermediary that comes between it and me. This intermediary has been given the term “sense datum” and the plural “sense data.”

This argument can be applied not just to far distant objects, but to everything we perceive. Light also takes time to travel from the cup to my eyes. Therefore, I am now perceiving the cup as it was a fraction of a millisecond ago. As said, in extreme cases the objects of perception may no longer exist at the moment when the causal process of perception is complete. One should, therefore, accept that all the events we perceive are to some extent in the past.

The argument from illusion seemingly shows the need to posit sense-data as the immediate objects of perception. In cases of illusion or hallucination, the object has qualities that no public physical object in that situation has and so must be distinct from any such object.

Indirect realism invokes the veil of perception. All we actually perceive is the veil that covers the world, a veil that consists of our sense data. What, then, justifies our belief that there is a world beyond that veil? In drawing the focus of our perception away from the world and onto inner items, we are threatened by wholesale skepticism. Since we can only directly perceive our sense data, all our beliefs about the external world beyond may be false. There may not actually be any coffee cups or olive oil tins in the world, merely sense data in my mind.Some have embraced the skepticism suggested by indirect realism and accepted the anti-realist position that there is no world independent of the perceiver. This position is known as idealism.