1. What are you most certain of and how do you know it? Let’s say you’re pretty sure about many things but most certain about mathematical truths (e.g., an equilateral triangle has three 60 degree angles). Let’s say to prove this you draw a triangle on the chalk board. But the triangle on the chalk board is imperfect: the lines are grainy and not quite straight, etc. In fact, every physically produced triangle will fall short of the perfect triangle. But all of your mathematical knowledge about triangles is based on the perfect triangle—the one in which the lines are perfectly straight and the angles exactly what they should be. That ideal triangle, the one that doesn’t exist in the material world, is the only one you really know anything about, but luckily that knowledge carries over, albeit imperfectly, in the material world. And mathematical knowledge, being the most certain, is a model for other kinds of knowledge. So according to this theory, the material world does exist, but for us to have any real knowledge about it requires assumption of an ideal world that transcends this or that material object.
2. The second critique disagrees with the first, and assumes that knowledge is sensory based. All knowledge begins with the five senses. But what knowledge do you gain from the five senses? Knowledge about the material world? Not at all. “The pillow is red” is not a proposition about the pillow itself, but about how the pillow registers in the retina. “The pillow is soft” says nothing about the pillow in itself but only addresses how the pillow registers tactile sensations in neurons under the skin. In fact, we may agree that the pillow is largely empty space with tiny atoms bombarding each other. But this knowledge too has been derived from empirical studies that rely in the first instance on sensory data. So all we know about are the imprints made upon our own body’s sensory registers. If there are material things out in space which exist independently of our sensory judgments, we can know nothing about them in themselves—we can only know about them post-processed, as it were. So according to this theory, the material world may exist, but we’ll never know, because all we know about are the products of our own subjective processing plant.
The first critique was favored in the 4th century BC by Plato, who deliberately rejected empiricism (and the Greeks had already theorized that the world was nothing more than billions of tiny particles called atoms bombarding one another and creating the appearance of solid shapes—a conclusion they drew through philosophical inquiry without any need for scientific instruments, and a materialist conclusion Plato had studied and rejected on the grounds that it could only give us low-level knowledge accessible to the senses, knowledge of material reality, which is only a shadow cast by a more ideal reality accessible to reason).
The second critique was favored by the empiricist, David Hume, who was presumably following the empiricism of 18th-century scientists to its logical conclusion—that the material world was either non-existent or utterly unknowable. (There are counterarguments—and rebuttals—Kant, e.g., would pick up exactly where Hume left off. But it’s interesting to see how the ultimate empiricist, Hume, and the ultimate anti-empiricist, the rationalist Plato, both conclude that a purely materialist world view is untenable.)