Tree of Knowledge
"Referred to in the book of Genesis, it is the tree in the Garden of Eden whose fruit God forbade Adam and Eve to eat. As used in the text (of The Prism of Lyra), the tree of the knowledge of good and evil symbolizes the knowledge of polarity." (PoL).
In verses 2 and 3, the book of Genesis mentions two trees, the tree of knowledge of good and evil, and the tree of life. The fruit of the tree of knowledge was the apple.
There are various interpretations of the meaning of the expression 'good and evil'. Keith Priest and Lyssa Royal, e.g., interpret it quite literally, as the knowledge of polairty, whereas many scolars refer to it as a common Egyptian expression, meaning everything. The tree of knowledge would therefore have been the tree of all knowledge.