implies that we see some of its parts but not others. It is necessarily known that the aforementioned conditions are met with respect to the parts that are not seen just as they are met with respect to those that are seen. Since only some parts of things meeting all these conditions are seen, rather than all their parts, we know that vision does not necessarily occur when these conditions are met.

The second line of argument was that if perception necessarily occurred when these conditions were met, we would necessarily see indivisible atoms and dust motes. But this consequent is plainly false, so the antecedent is likewise false. The implication is established as follows: when we see a large body, we must see a small part of it, for if we did not see any small part, we would not see the whole composed of those parts. Moreover, seeing one part does not depend on seeing another, since that would entail circularity. It must therefore be possible for us to see an indivisible atom.

If these two arguments are unsound, they are open to criticism; if they are sound, they show that vision does not necessarily occur in the presence of