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.
Aristotle's concept of hyle is the principle that correlates with eidos (form) and this can be demonstrated in the way the philosopher described hyle, saying it is that which receives form or definiteness, that which is formed. Aristotle explained that "By hyle I mean that which in itself is neither a particular thing nor of a certain quantity nor assigned to any other of the categories by which being is determined." This means that hyle is brought into existence not due to its being its agent or its own actuality but only when form attaches to it. It has been described as a plenum or a field, a conceptualization that opposed Democritus' atomistic ontology. It is maintained that the Aristotelian concept should not be understood as a "stuff" since there is, for example, hyle that is intellectual as well as sensible hyle found in the body.
For Aristotle, hyle is composed of four elements – fire, water, air, and earth – but these were not considered pure substances since matter and form exist in a combination of hot, moist, dry, and cold so that everything is united to form the elements.
The Latin equivalent of the hyle concept – and later its medieval version – also emerged from Aristotle's notion. The Greek term's Latin equivalent was silva, which literally meant woodland or forest. However, the Latin thinkers opted for a word that had technical sense instead of the literal meaning so that it became understood as that of which a thing is made but one that remained a substratum with changed form. The word materia was chosen instead to indicate a meaning not in handicraft but in the passive role that mother (mater) plays in conception.
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). For the neo-Aristotelian theorists, a corporeal substance is a hylemorphic composite, which means it is a combination of primary matter and a substantial form.
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.[original research?] 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.