Baruch “Benedictus” de Spinoza (he used the name Benedictus for most of his scholarly writings) lived from 1632 to 1677. Spinoza was born in Amsterdam to a Portuguese Jewish family, and (like the character of Ester) educated in a devout Jewish tradition. As a young man, Spinoza began to study with Franciscus van den Enden, a philosopher and former Jesuit priest (van den Enden is also featured as a character in The Weight of Ink). After being exposed to philosophical works, including writings by the French Enlightenment philosopher Descartes, while studying with van den Enden, Spinoza began to explore more controversial and radical beliefs. He publicly questioned Jewish teachings about the Torah and the nature of God (although he never seems to have outright challenged belief in God, or advocated atheism).

In 1656, when Spinoza was only 23 years old, he was formally excommunicated from the Jewish community due to his unorthodox beliefs. Kadish references these events in her novel, adding fictional details about Rabbi Mendes (a fictional character) advocating on behalf of Spinoza. After being expelled from the Jewish community, Spinoza never converted to Christianity. He continued to live in Amsterdam, focusing on writing philosophy and fashioning lenses for microscopes.