A representation may be defined simply enough (as Barfield does in Saving the Appearances) as "something I perceive to be there" (SA 19).

But our perception of even the simplest representation is no simple affair, as Barfield explains:

For the conversion of the unrepresented into a representation, at least one sentient organism is as much a sine qua non as the unrepresented itself; and for the conversion of the unrepresented into representations even remotely resembling our everyday world, at least one nervous system organized about a spinal cord culminating in a brain, is equally dispensable. (SA 22)

"Every fact," Goethe insisted, "is already theory." Barfield, too, takes pains to remind us that a great deal of mental activity goes into the construction of even the simplest "fact."

We "perceive," as we say, an "object," but the object is already "nature" . . . that is to say, a construct owing, not to our senses but to our minds its total form, and thus its unity, and thus its presence as an object. A single percept, uncombined, unmodified and unadopted by any element of mental construction, is as much outside our normal experience as is pure thinking, uncombined with any element of representation borrowed from the senses. (WCT 18)

See in particular Saving the Appearances, passim.