In a philosophical sense, substance is contradistinguished from accident, and signifies that which exists independently and unchangeably; whilst accident denotes the changeable phenomena in substance, whether these phenomena are necessary or casual, in which latter case they are called accidents in a narrower sense. Substance is, with respect to the mind, a merely logical distinction from its attributes. We can never imagine it, but we are compelled to assume it. We cannot conceive substance short of its attributes, because those attributes are the sole staple of our conceptions; but we must assume that substance is something different from its attributes. Substance is the unknown, unknowable substratum on which rests all that we experience of the external world.
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