Final answer:
The apple in "A Poison Tree" alludes to themes of original sin, temptation, revenge, and corruption of human nature, drawing parallels to the biblical story of Adam and Eve and the apple from the Tree of Knowledge.
Step-by-step explanation:
In William Blake's poem "A Poison Tree," the apple symbolizes multiple intertwined themes which include original sin from the Garden of Eden, the temptation to commit a harmful act, the cycle of revenge, and the loss of innocence and the corruption of human nature. The apple alludes to the biblically infamous fruit from the Tree of Knowledge of Good and Evil, a narrative touchstone that represents the fall of man and the initial transgression that led to the human understanding of right and wrong.
The poem reflects on the destructive effects of supressing anger towards a foe resulting in deceitfulness that leads to the foe's demise. Thus, just as Eve's consumption of the forbidden fruit led to severe consequences for mankind, the speaker's nurturing of wrath bears fruit in the form of an apple that ultimately poisons the foe. The apple in the poem serves as a powerful metaphor for the outcomes of unchecked emotions and secretive vengeance.