In those days the sun was supposed to be only a great fire; a lightning-flash, an aurora, or a comet, was merely a flame; in other words, the essence of all these was the element fire, or, as it was later called, caloric. The sun, except when he appeared as the spreader of pestilence, was the beneficent fire, as were also some of the planets; the lightning, the comet, even the moon and Saturn, were baleful fires. This endeavour to assign a substantive existence to every phenomenon is, of course, perfectly natural; but on that very account excessively likely to be wrong. _Humanum est errare_ comes with quite as much heart-felt conviction of its truth from the lips of the honest Pagan as from those of the Christian believer; though perhaps its meaning may be considerably less extended in the former than in the latter case. 94. But, before discussing what is that _something else_ besides stuff which has an _objective_ though not a _substantive_[35] existence, let us in the first place inquire into the grounds of our belief, that matter itself has a real existence external to us; that, in fact, the so-called evidence of our senses is not a mere delusion. There is a strong temptation to be metaphysical here, but we will endeavour to resist it. Now physical science furnishes us with the following among many other arguments in proof of the reality of the external universe:— _Experience of the most varied kind consistently shows us that we cannot produce or destroy even the smallest quantity of matter._ Exercise our greatest powers of imagination, do with it what we please, we cannot make our senses indicate to us an increase or diminution in a given quantity of what we call matter. We find it so far amenable to our control that we can alter its arrangement, form, density, state of aggregation, temperature, etc.; nay, by so approximating it to other matter as to produce a chemical combination, we may entirely transform its appearance and properties,—_all but one_: its mass or quantity is completely beyond our control. Measure it by what process we please, by the ‘muscular sense,’ by weight, anyhow, there it is, altogether independent of us, laughing our efforts to scorn! Can this be a mere mental idea which the mind that conceived it (or, at all events, in some way received the conception of it) is unable to destroy? But there is one other argument on this point which must be mentioned. Not only do our own senses invariably indicate to us the impossibility of altering the quantity of matter, but the senses of all men alike point to the same quantity, quality, and collocation of matter in the earth and external to the earth. Whence this extraordinary agreement between the evidences of the senses in different men, when the minds are so different?