That which occupies space, possessing size and shape, mass, movability, and solidity (which may be the same as impenetrability). Its nature was historically one of the great subjects of philosophy, now largely pursued through the philosophy of physics. Plato and Aristotle passed on a classification of matter into four kinds (earth, air, water, and fire) but also the view (not necessarily held by Aristotle himself) that any such division reflected a different form taken by one prime, undifferentiated matter or hylē (see materia prima). In Aristotle there is also a fifth kind of matter (quintessence) found in the celestial world, whose possessors were thereby exempt from change. This physics was replaced from the 17th century onwards by the classical conception first of corpuscles (see corpuscularianism) and then of modern atoms. In modern physics, the tidy picture of inert massy atoms on the one hand, and forces between them on the other, has entirely given way. The quantum mechanical description of fundamental particles blurs the distinction between matter and its energy, and between particles and the forces that describe their interaction. Philosophically, however, quantum mechanics leaves considerable unease of its own.