material thing which we behold, occupies considerably more space than it fills. This is proved by the consideration that everything admits of compression. All sensible matter, therefore, is porous; dense as some kinds of it may appear, the particles even of the most compact matter are never actually in contact, consequently all bodies occupy more space than they fill, or perhaps we should rather say, appear to fill more space than they actually do fill. All matter is interspersed with vacant cavities or interstices. The atom alone has no such interstices; it alone fills actually the same space in which it is.
3. The atom, then, besides being the absolutely least, is also the absolutely full, while the interval between atom and atom is the absolutely void, empty space. Empty space is thus the supplementing conception which the Atomic philosophers conjoin with their conception of the atom. What Being and not-Being were to Heraclitus, the full and the empty (τὸ πλῆρες καὶ τὸ κενόν), or atoms and the void, were to the Atomists. These (the full and the empty) were the principles of their system; and out of these they conceived that the constitution of the universe, and all the appearances which it presents to our senses, might be explained.
4. Another consideration to be kept particularly in view in studying this system is, that the atoms
