Why Spinoza never completed the Treatise on the Intellect

We may suppose, then, that the discovery of the common notions occurs precisely at the end of the edited part of the Treatise, and at the beginning of the writing of the Ethics: in about 1661 – 1662. But why would this discovery have forced Spinoza to abandon the already-existing version of the Treatise? The explanation is that the common notions emerge at a time when they cannot fulfil their functions or develop their consequences. They are discovered too late relative to the text of the Treatise. They would have to establish a new point of departure for philosophy; but the point of departure has already ben set in geometric ideas.
They would have to determine an adequate mode of knowledge of what exists, and show how one passes from this mode of knowledge to the ultimate mode, knowledge of essences. But because the modes of knowledge have already been defined in the Treatise, there is no place left for the common notions or for the series of fixed and eternal things, which are thus shifted over to the ultimate mode of knowledge, with the knowledge of essences. In short, in order to give the common notions their place and function, it would have been necessary for Spinoza to rewrite the entire Treatise. It is not only that they invalidate the finished part, but they would have modified it. Spinoza prefers to write the Ethics form the perspective of the common notions, although it means postponing a new treatise that would have focussed on the practical problems that are merely outlined at the end of the Ethics, concerning the origin, the formation, and the series of these common notions, along with the corresponding experiment.
Deleuze – Spinoza: Practical Philosophy, p. 120 – 121

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