6. As to causation in the second sense (viz. an ordinary succession of antecedents and consequents, or what is called the Order of Nature), when so explained, it falls under the doctrine of general laws; and of this I proceed to make mention, as another first principle or notion, derived by us from experience, and accepted with what I have called a presumption. By natural law I mean the fact that things happen uniformly according to certain circumstances, and not without them and at random: that is, that they happen in an order; and, as all things in the universe are unit and individual, order implies a certain repetition, whether of things or like things, or of their affections and relations. Thus we have experience, for instance, of the regularity of our physical functions, such as the beating of the pulse and the heaving of the breath; of the recurring sensations of hunger and thirst; of the alternation of waking and sleeping, and the succession of youth and age. In like manner we have experience of the great recurring phenomena of the heavens and earth, of day and night, summer and winter. Also, we have experience of a like uniform succession in the instance of fire burning, water choking, stones falling down and not up, iron moving towards a magnet, friction followed by sparks and crackling, an oar looking bent in the stream, and compressed steam bursting its vessel. Also, by scientific analysis, we are led to the conclusion that phenomena, which seem very different from each other, admit of being grouped together as modes of the operation of one hypothetical law, acting under varied circumstances. For instance, the motion of a stone falling freely, of a projectile, and of a planet, may be generalized as one and the same property, in each of them, of the particles of matter; and this generalization loses its character of hypothesis, and becomes a probability, in proportion as we have reason for thinking on other grounds that the particles of all matter really move and act towards each other in one certain way in relation to space and time, and not in half a dozen ways; that is, that nature acts by uniform laws. And thus we advance to the general notion or first principle of the sovereignty of law throughout the universe.