HoPwaG: Democritus and Leucippus

Atomists' idea: that all bodies made of particles--atoms--invisible to the eye, and observable phenomena explained by their interaction. Etymology: a- not, tom- cutting: an atom can't, by its nature, be cut. (Of course, our atoms can be cut.) Democritus leaves us fragments about ethics as well as Atomism.

Leucippus, Democritus, et al get to Atomism by argument, not experiment:

• Recall Zeno: if distances or bodies can be divided infinitely, get paradoxical conclusion.
• The Atomist response: once reach atoms, can't keep cutting.

• Recall Melissus on motion: motion requires empty space, which by (PPMI) is ruled out;
• The Atomist response: since plainly motion is possible, there must be nothingness, i.e. a void.

Putting those conclusions together, we get a cosmological picture in which atoms move in nothingness. Note: this is not quite a picture in which atoms move in empty space, where space is something different from non-being. The Atomists literally mean that atoms (what has being) move in a void, i.e. in non-being, i.e. atoms replace non-being (void) with being (atoms) when they move.

Details of the Atomist picture:

• Something like our intuitive picture of 'microscopic ball bearings' bouncing off each other and conglomerating.
• But complications: some have different shapes, to "hook together" in different ways.
• Explain vision by sheets of atoms being shed from outer layers of bodies

Another Atomist doctrine: there is an infinite number of atoms. Note that their difference from the Eleatics can be then summarized as: being is manifold and infinite, separated by void, as opposed to the Eleatic doctrine that being is one and infinite, with no separation. That is, the Atomists' atoms are microcosmic versions of Parmenides' being.

Argument for infinity of atoms:

• Principle of Sufficient Reason (PSR): no reason for any finite number of atoms, similarly no reason any shape should be lacking.

Objection:

• If there are infinite number, and by PSR no reason for any particular number, size, shape, etc., then some atoms should be visible.

Possible Atomist reply:

• Such atoms would be invisible anyway, by theory of vision above, since they can't send off sheets of atoms;

The Atomists' cosmological story:

• collisions and entanglements of atoms of different degrees of heaviness explains, e.g., planets, which are conglomerations of the heavier stuff.

Democritus gets skeptical conclusions from Atomist doctrines. Since atoms are all there really is, and nothing sensible is an atom, the senses are not sources of knowledge (expressed in his 'by convention' slogan). So Atomism not necessarily a defense of common sense against, e.g., Parmenides.

Note that Democritus gets to this kind of skepticisim by virtue of the Eleatic rational argument approach to cosmology. What we get by rational argument undermines what we get by sensory experience. That's as opposed to contemporary atomic theory which explains and vindicates sensory experience, and thus puts no pressure on the reality of, e.g., mid-sized dry objects.