Spinoza engaged in correspondence from December 1664 to June 1665 with Willem van Blijenbergh, an amateur Calvinist theologian, who questioned Spinoza on the definition of evil. Later in 1665, Spinoza notified Oldenburg that he had started to work on a new book, the "Theologico-Political Treatise", published in 1670. Leibniz disagreed harshly with Spinoza in his own manuscript "Refutation of Spinoza", but he is also known to have met with Spinoza on at least one occasion (as mentioned above), and his own work bears some striking resemblances to specific important parts of Spinoza's philosophy (see: Monadology).