Polarity holds that opposites are one thing at different degrees. Heat and cold are a single scale read up or down, and so, the book says, are love and hate, courage and fear. The consequence it wants, and the reason the principle is there at all, is that a state can be moved along its own scale rather than replaced by its contrary.
Antecedents are not hard to find. Heraclitus has the road up and the road down as one road. The alchemical literature works constantly with the reconciling of opposites, and Nicholas of Cusa gave that idea a name in the fifteenth century. What The Kybalion adds is a flattening: every pair is treated as one scale, without asking whether every pair really is one.
That is where the principle is weakest, and the atlas records it as contested rather than settled. Temperature genuinely is a single scale. Truth and falsity are not, and treating them as poles of one thing yields the conclusion that any statement is true to a degree, which the book does not shrink from. Readers within the Hermetic tradition have divided on this for a century. Some take Polarity as a psychological technique wearing a physical metaphor, useful precisely because moods do behave like a scale. Others take it as a metaphysical claim about how reality is built, and accept the consequences for truth. The atlas does not settle the question and does not need to.