Since Witmer brought up the idea of Platonism, and since Daniel Morgan [see update below] has also claimed to be a Platonist when it comes to the laws of logic, I thought a brief illustration of what Platonism is would be in order before continuing with my response to Witmer.Platonism is, simply, the idea that there are universal forms (or archetypes) that exist as abstractions, and material objects are “shadows” of those universals. As shadows, they don’t correspond 100% to the universals, and thus there are variations.

For example, suppose a red fire truck drives past you. There is a universal form of “redness” that the color of the fire truck is a shadow of. Likewise, there is a universal form of “fire truck”ness that the fire truck corresponds to. Or to take another illustration, if you see a tree, it is but a representation of the abstract treeness. It’s green leaves are shadows of the universal greenness and universal leafness.

In other words, all material objects are merely “copies” of the archetypical forms for each object. As such, Plato argues that the copies are not true knowledge. Instead, to find true knowledge, one must consult the archetypes. Thus, Plato becomes an idealist (arguing that the immaterial forms hold primacy over materialistic instances of those forms).

Platonism cannot coexist with materialism. Materialism is the idea that everything either is, or reduces to, physical objects, causes and effects. Plato, on the other hand, argues that these physical objects (and by extension their causes/effects) are shadow-forms of an archetypical form that is immaterial.

And herein lies the problem for the materialistic atheist who would seek to invoke Plato to avoid the Presuppositional argument. He is, at this point, stating that the immaterial holds primacy over the material world; and as such, he cannot be a materialist anymore. In short, he refutes his own worldview.

For further research:

http://en.wikipedia.org/wiki/Platonism
http://www.english.hawaii.edu/criticalink/plato/terms/forms.html

UPDATE: in his latest comment, Morgan said: “BTW, in my earlier comment regarding Platonism, I want you to note that I am not actually advocating Platonism, but asking you if it is necessarily contradictory to frame the ontological conditions as I have…”