Well, if this is Philosophy 201, then clearly everything is abstract, and something something Plato's cave. However, the default position is that (surprise!) ob

Well, if this is Philosophy 201, then clearly everything is abstract, and something something Plato's cave. However, the default position is that (surprise!) objective reality exists. Things that are observable and measurable are not abstract. Those things around you that you can perceive with your senses are not abstract. For a more in-depth perspective, you could take a look at the Wikipedia pages for: empiricism, epistemology, philosophy of science. Also, the Stanford Encyclopedia of Philosophy should have a reasonably topical article about Rationalism vs. Empiricism.