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In philosophy, hyle (Template:PronEng; from Template:Lang-grc) refers to matter or stuff. It can also be the material cause underlying a change in Aristotelian philosophy. The Greeks originally had no word for matter in general, as opposed to raw material suitable for some specific purpose or other, so Aristotle adapted the word for "timber" for this purpose. The idea that everything physical is made of the same basic substance holds up well under modern science, although it may be thought of more in terms of energy or matter/energy.
The idea of hyle is closely related to that of substance, in that both are that which remains unchanged during any process of change. There is cause for criticism of the very postulation of its existence, for many of the reasons that Nietzsche was critical of Kant's "Ding an sich", or "thing in itself", which is generally described as whatever is its own cause, or alternately as a thing whose only property is that it is that thing (or, in other words, that it has only that property).
There is no way to directly prove the existence of any thing which has no properties, since such a thing could not possibly interact with other things and thus would be unobservable and indeterminate.
On the other hand, we may need to postulate a substance that endures through change in order to explain the nature of change—without an enduring factor that persists through change, there is no change but only a succession of unrelated events. (See hylomorphism). The existence of change is hard to deny, and if we have to postulate something unobserved in order to explain what is observed, that is a valid indirect demonstration (by abductive reasoning). Moreover, something like a prime substance is posited by physics in the form of matter/energy.
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