Walter Ong characterizes the main differences between the languages of oral and literate cultures in these terms:

[It] is possible to generalize somewhat about the psychodynamics of primary oral cultures, that is, of oral cultures untouched by writing. … Fully literate persons can only with great difficulty imagine what a primary oral culture is like, that is, a culture with no knowledge whatsoever of writing or even of the possibility of writing. Try to imagine a culture where no one has ever ‘looked up’ anything. In a primary oral culture, the expression ‘to look up something’ is an empty phrase: it would have no conceivable meaning.

Without writing, words as such have no visual presence, even when the objects they represent are visual. They are sounds. You might ‘call’ them back—’recall’ them. But there is nowhere to ‘look’ for them. They have no focus and no trace (a visual metaphor, showing dependency on writing), not even a trajectory. They are occurrences, events.