Note (return) — Chapter XIII

And now the grand crucial question arises--How do atoms combine so as to form concrete bodies? If they move in straight lines, and with equal rapidity from all eternity, then they can never unite so as to form concrete substances. They can only coalesce by deviating from a straight line. 798 How are they made to deviate from a straight line? This deviation must be introduced arbitrarily, or by some external cause. And inasmuch as Epicurus admits of no causes "but space and matter," and rejects all divine or supernatural interposition, the new movement must be purely arbitrary. They deviate spontaneously, and of their own accord. "The system of nature immediately appears as a free agent, released from tyrant masters, to do every thing of itself spontaneously, without the help of the gods." 799 The manner in which Lucretius proves this doctrine is a good example of the petitio principii. He assumes, in opposition to the whole spirit and tendency of the Epicurean philosophy, that man has "a free will," and then argues that if man who is nothing but an aggregation of atoms, can "turn aside and alter his own movements," the primary elements, of which his soul is composed, must have some original spontaneity. "If all motion is connected and dependent, and a new movement perpetually arises from a former one in a certain order, and if the primary elements do not produce any commencement of motion by deviating from the straight line to break the laws of fate, so that cause may not follow cause in infinite succession, whence comes this freedom of will to all animals in the world? whence, I say, is this liberty of action wrested from the fates, by means of which we go wheresoever inclination leads each of us? whence is it that we ourselves turn aside, and alter our motions, not at any fixed time, nor in any fixed part of space, but just as our own minds prompt?.... Wherefore we must necessarily confess that the same is the case with the seeds of matter, and there is some other cause besides strokes and weight [resistance and density] from which this power [of free movement] is innate in them, since we see that nothing is produced from nothing." 800 Besides form, extension, and density, Epicurus has found another inherent or essential quality of matter or atoms, namely, "spontaneous" motion.