In philosophy, hyle (/ˈhaɪliː/; from Ancient Greek: ὕλη) 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 "wood" to 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.
Substance
The matter of hyle is closely related to that of substance, in so far as both endure a change in form, or transformation. Aristotle defined primary substance as that which can neither be predicated nor attributed to something else, and he explained the transformation between the four terrestrial elements in terms of an abstract primary matter that underlies each element due to the four combinations of two properties: hot or cold and wet or dry. He stipulated that transformations between opposing elements, where both properties differ, must be analyzed as two discrete steps wherein one of the two properties changes to its contrary while the other remains unchanged (see essence and hylomorphism).
Modern substance theory differs, for example Kant's "Ding an sich", or "thing in itself", is generally described as whatever is its own cause, or alternatively as a thing whose only property is that it is that thing (or, in other words, that it has only that property). However, this notion is subject to the criticism, as by Nietzsche, that 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. 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.