Extension (metaphysics)
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In metaphysics, extension is, roughly speaking, the property of "taking up space". René Descartes defines extension as the property of existing in more than one dimension. For Descartes, the primary characteristic of matter is extension, just as the primary characteristic of mind is consciousness.
For John Locke, extension is "only the space that lies between the extremities of the solid coherent parts" of a body. It is the space possessed by a body. Locke refers to the extension in conjunction with 'solidity' and 'impenetrability,' the other primary characteristics of matter.
Extension also plays an important part in the philosophy of Spinoza, who claims that substance (that which has extension) can only be limited by substance of the same sort, i.e. matter cannot be limited by ideas and vice versa. From this principle, he determines that substance is infinite. This infinite substance is what Spinoza calls God, and it possesses both unlimited extension and unlimited consciousness.
The property of extension has not played a significant role in philosophy roughly since the time of Kant. Kant maintained a distinction between the mind and the body, differentiating space as the realm of the body and time the realm of the mind. He makes no mention of "extension," however, and no philosophers have dealt extensively with the topic since Kant's writing.
Metaphysics have largely disappeared from contemporary philosophy, rendering extension as a term reserved for historical inquiry.