to decide or even to guess whether this idea will or will not pass with credit some of the most elementary examinations to which a theory of the ultimate nature of matter must of course be subjected. 135. Take them for what they are worth. The four forms of speculation we have just sketched represent the most plausible guesses yet propounded as to the ultimate nature of matter, the second being probably because the most artificial and the most arbitrary, the most completely developed. For in it the representation is self-contained as it were; it does not base itself upon extraneous postulates, as of ultimate hard particles (of what?), nor upon vortex motion (of what? again), nor, finally, upon mere intense heterogeneity (of what? once more), as do the other three. But we naturally object to it as refining away altogether the idea of _stuff_ or _substance_ which the mind seems to require as something underlying the notion of anything which is found to be directly capable of affecting our senses. 136. The reader who has followed us so far, must now see that our notions of the nature of matter are, at best, but hazy. We know, it is true, a great many of its properties very exactly, so much so indeed, as to be able to deduce from them mathematically an immense variety of consequences which subsequent experiment shows to be correct, at least within the limits of accuracy of our methods of observation and measurement. But as to _what it is_ we know no more than Democritus or Lucretius did, though as to what it may be or may not be we are perhaps considerably better prepared with an opinion than they could possibly be. 137. We have seen in the preceding chapter that energy is never found separate from matter, so that we might, with perfect propriety, define matter as the seat or vehicle of energy—that which is essential to the existence of the known forms of energy, without which, therefore, there could be no transformations of energy, and therefore no _life_ such as we now know it. 138. The transformability of a given amount of energy, or, at least, the mode of its transformation, often depends in a very curious manner upon the relative quantity of matter with which it is associated. We have already seen this in the case of heat. For, when a given quantity of heat is associated with a small quantity of matter, it is at a high temperature, and has great availability, but its temperature, and therefore its availability, become lower as the quantity of matter with which it is associated is increased. It is possible that radiant heat and light owe their high availability to the very small density of the luminiferous ether.