Man is not truly one, but truly two.
— Dr Jekyll, The Strange Case of Dr Jekyll and Mr Hyde · Robert Louis Stevenson
A human being isn't a single self — we're really two selves at once.
Jekyll's theory is the engine of the whole story: that respectable and monstrous selves live in one body, and that the civilized half spends its life suppressing the other. His experiment doesn't create Hyde — it just sets free what was always there.