Taken from Ontology for beginners https://philosophynow.org/issues/16/Ontology_for_Beginners
RealismThis is the view that physical objects exist even when nobody is watching them. In other words the external world exists independently of us. This is of course the common sense view, but it is a view that has been seriously attacked by philosophers known as idealists (see below)
IdealismIn philosophy (as opposed to politics), an idealist is not someone who wants to change the world but someone who thinks the world is a creation of the mind; either of the mind of the beholder, or in Berkeley’s case, of the mind of God. Oversimplifying a bit, idealists think that the whole shooting match (the world, it’s inhabitants, plant-life, stars, geology, books, everything) is all some sort of strange exotic dream.
Brain in a VatThis is a popular modern illustration of a type of idealism. You think you are wandering around the world, seeing, feeling, talking to people. But suppose that, following a horrific accident, the doctors removed your brain, put it in a vat of nutrient fluids and kept it alive. Suppose that to keep you sane they hooked up the severed nerve endings of your brain to some very expensive electrical equipment and then fed you with signals that your brain interpreted as sights, sounds, smells and so on. The question is, would you know you were just a brain in a vat at all, if the illusion was perfect, and they didn’t tell you? How would you know? If you couldn’t know, then how do you know that it hasn’t already happened? And so, how do you know that the world out there actually exists?
SolipsismThis is the theory that the world out there doesn’t exist. The whole thing is a product not just of a mind (idealism), but specifically of your mind. The rest of us are just figments of your imagination. Few people seriously believe solipsism to be true, but it is a remarkably difficult theory to disprove. You’ll find that anything that you can call up as evidence against solipsism could be part of the illusion.
RelativismRelativism is the idea that there are many equallyvalid ways of seeing the world. This is quite compatible with the idea that there is in fact a single, independently-existing world out there, but like a mountain, the way it looks depends on where you are standing (see ‘A Place for Relativism’, on p.17). The most popular form of relativism these days is postmodernism.
PostmodernismFor postmodernists, ‘reality’ is what seems to you to be real; the environment in which you exist and make decisions. However, the way things appear to you will depend on the nature of your personal perceptive organs (you might be colourblind, or deaf, or whatever) and also of the assumptions of the culture in which you grew up. Your world, then, is a ‘culturally-constructed interpretation’. All we can say with confidence about what you regard as reality is that it seems real to you. If you think you see a giant hedgehog following you around, then the rest of us shouldn’t be judgmental; the giant hedgehog is part of your reality.
Immanuel KantIs widely regarded as having hit the nail on the head when he distinguished between the noumenal world (the world as it really is, independently of our perceptions) and the phenomenal world (the sum of all our perceptions).
PhenomenologyPhenomenologists, following on from Kant’s distinction above, studied our perceptions in great detail, attempting to separate the raw sense-data (light, sounds, colours, shapes etc) from the interpretations which our minds early-on put on that data (that’s a chair; the sun’s coming up; she’s pretty, etc). They wanted to study the former without the distortions created by the latter in the hope of coming to a better understanding of what the noumenal world is like.
POSTSTRUCTURALISM - TAKEN FROM WIKIPEDIA (UNPARAPHRASED)
The author's intended meaning is secondary to the meaning that the reader perceives. Also the author's identity as a stable "self" with a single, discernible "intent" is a fictional construct. Post-structuralism rejects the idea of a literary text having a single purpose, a single meaning, or one singular existence. Instead, every individual reader creates a new and individual purpose, meaning, and existence for a given text.
A post-structuralist critic must be able to use a variety of perspectives to create a multifaceted interpretation of a text, even if these interpretations conflict with one another. It is particularly important to analyze how the meanings of a text shift in relation to certain variables, usually involving the identity of the reader (for example: class, racial, or sexual identity)