Descartes defined matter in terms of extension. It could just as well be defined in terms of time, for both the one and the other are external condit… - Benedictus de Spinoza

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Descartes defined matter in terms of extension. It could just as well be defined in terms of time, for both the one and the other are external conditions of its existence in spatial and temporal relations. Thus both become also the necessary conditions of measurement for all thinking minds, which are themselves limited by place and time, but they never become the essence of matter. Spinoza struggled for a long time against this Cartesian explanation, probably because he felt that something about it was not clear. He was not satisfied with his teacher's sharp distinction between matter and spirit, but, since he lacked a unifying intermediate conception, what could he do? Unfortunately then, in his Ethics he still took matter for extension, that is space, and set it up beside thought, an entirely different kind of thing. Now he was indeed on the way to a very intricate confusion.