Throughout the book, the motif of vision is used to underscore ideas of cultural arrogance and understanding. Nathan's inability to see outside of his narrow world view, in particular, is evoked repeatedly with images of blindness and poor sight. His one physical war wound from World War Two is a damaged eye, which leaves him with poor vision on the left side. This wound can be seen as the bodily manifestation of his more serious psychological or spiritual wound, which is another sort of blindness, a metaphorical blindness to anyone or anything outside of himself and the conception of his divine mission. His good right eye is temporarily damaged as well, when he arrogantly ignores Mama Tataba's advice and continues grappling with the poisonwood tree. When Leah finally sees her father clearly, as the cruel, delusional man he is, she notices that "his blue eyes had a vacant look" (Bel and the Serpant: Leah).
Adah, by contrast, who alone among the Prices never views the Congolese as inferior to Westerners, has amazingly agile vision. She is able to see words backwards and forwards equally well, ringing double meaning from any phrase.
In Joseph Conrad's the The Heart of Darkness, the darkness of the title is located in Africa itself, and also in the heart of those who are forced to acclimate to its primeval and brutal environment. Throughout The Poisonwood Bible Kingsolver challenges this notion, playing with the themes of darkness and light to get us to reconsider where these forces really reside. In Nathan's first sermon in Kilanga he repeatedly uses the biblical phrase, "nakedness and darkness of the soul" to refer to the natives' shameful state of undress, but tellingly, the one time that the phrase "heart of darkness" is used is in describing Nathan (The Revelation: Orleanna). Darkness is seen as emanating not from Africa, but from the Western oppression of the Africans, which Nathan represents. Africa, in fact, is portrayed as anything but primeval and brutal. The people of Kilanga are every bit as civilized as the Prices, and, if anything, far less brutal. If Africa is associated with darkness, it is only because it brings out the darkness of greed and hubris in the hearts of men like Nathan and Eeben Axelroot. Africa itself, rid of brutal Western interference, can become, in Patrice Lumumba's words "the heart of light."
The phrase, "walk forward into the light" appears twice in the book, and with two very different meanings that play off one another nicely. The first time it is used is when Nathan walks around his daughter's dead body, using the rain to baptize the local children gathered there in mourning. Leah describes him here as "imploring the living progeny of Kilanga to walk forward into the light" (Bel and the Serpent: Leah). The light he is imploring them to walk into is, of course, the light of Christianity—the light of his own firmly-held religious beliefs that he would like to impose on them. In the very last line of the book, Ruth May addresses her mother with these same words. Here the light refers not to Christianity, but to forgiveness, and a world devoid of the darkness that blackened the hearts of men like Nathan Price.
Each of the Price daughters has a distinctive relationship to language. Rachel consistently and unapologetically misuses words, Adah reads them backward, Ruth May cheerfully invents her own language in which to communicate with the local children, and Leah uses language lessons as an excuse to spend time with her future husband, Anatole. Each of these linguistic personalities mirrors the deeper personality of the girl: Rachel is self-involved, and wholly inward looking, ignoring the larger world around her; Adah is a brilliant and perceptive observer, seeing more in a glance than most could see in a lengthy examination; Ruth May is adventurous, confident, and playful; and Leah's relates to the world through her boundless capacity for love.
Lingala, the regional language used in the region of Congo that the Prices inhabit, also has its own linguistic personality. Most words in the language have wildly divergent meanings, and the intended meaning must be indicated by subtle differences in intonation. Adah is the first to pick up on this fact, loving the language for this feature, and Leah and Orleanna follow soon thereafter. Thick-skulled Nathan, however, never catches on, and therefore preaches every week that Jesus is a fatal Poisonwood Tree, when he means to declare that Jesus is dearly beloved.