Philosophy is the first, oldest, and largest discipline, leading to the founding of the first university by Plato and the formalization of logic and science by Aristotle, as well as spawning many other fields of study with recent major additions being psychology, sociology, the theories of mind used in AI research, and so on. Philosophy's affect on humanity's knowledge of the world is exemplified by things like xefer, which show that all articles on wikipedia are fundamentally predicated by the page for philosophy. Logic and critical thinking are sub-fields of philosophy, which explains why philosophy undergrads have the highest average scores on the LSAT and often the GRE too.

Philosophy concerns itself with the broadest and most general of questions. Philosophers do not study how rocks be or how living organisms be, as those are what geologists and biologists study, respectively. Instead the philosopher studies the nature of being itself, without specificity. Where all other disciplines are narrow in their scope and application, philosophy is broad and universally applicable. It is easy to argue that philosophy is the most practical of all studies, since a focus of philosophy is 'the good' and knowing what 'the good' is means you will know what is good to do in any particular situation.

This page was created for the 100th lecture of the Diogenesis Table Society.

Metaphysics is the study of being or existence and their scope such that we get clearly defined boundaries and properties for them. This includes space and time, causation, identity, etcetera. The focus on being or existence and the categorizations of its kinds is called 'ontology' and is the most lively sub-field of metaphysics.
  Many things are a matter of ontology. Ontological studies either go from definition to experience, or from experience to definition. For example, if we came across someone eating meat and claiming to be a vegetarian, then going from definition to experience, we would know the person was misinformed or confused. But if we went from experience to definition, then we have met a meat-eating vegetarian.
  Recent developments in metaphysics that I find noteworthy are Wilfrid Sellars' Myth of the Given, Max Black's Identity of Indiscernibles challenging Leibniz's Law, and Robert Nozick's closest continuer schema which resolves mereological problems of identity.

Epistemology is the study of knowledge, its scope and properties, whether there are individuated units of it, truth, justification, and the methods by which knowledge can be obtained. As a basic premise, units of knowledge are objects 'out there' in there world and not just illusory entities that exist solely in your mind. This is considered apodictic since resisting this premise results in solipsism.
  It is better to know than to not know. You could counter that knowing whether the number of air molecules in your room is even or odd obviously has no practical or meaningful use, but I would point out that the knowledge that would be required to determine this piece of information justifies its pursuit many times over, and that knowledge has near-endless practical and meaningful uses, making it at least better in capacity to know than to not know. Additionally, it's not very valuable to believe in information hazards because information is only ever hazardous if you don't know what you ought to do with it — a problem solved by gaining more information. The real hazard then is acting without thinking.
  Classically, knowledge is "justified true belief," as formalized by PlatoIt's worth noting that Plato also rejected similar formulations of knowledge like "true judgment with an account" in his work Theaetetus.. Something approximating this definition was in use until Gettier challenged the security of what would count as valid justification, also described as epistemic luck, but Plato already anticipated this objection and said that bad evidence isn't justification, so Gettier is mostly a non-problem despite the fame.
  It's useful to know the seven formal steps of the Socratic method, about deduction vs. induction vs. abduction, about problems with induction, epistemic naturalism, and about Wittgenstein's private language arguments (remarks 243-304, on page 48 in the PDF).

Logic is the study of correct thought (reason and argumentation), historically considered a sub-field of epistemology, logic as a discipline has become so large and so active that it is now considered its own field of philosophy. The study of logic within the constraints of validity and consistency is what is called formal logic (syntactical systems) and logic within the constraints of content and context is called informal logic (semantic systems).
  The principles of arguing well are logos, ethos, and pathos, which equates to logic, authority, and emotions, respectively. What this looks like in a real discussion:
   1. Don't say a contradiction.
   2. Don't interrupt people unless you need clarification.
   3. Add to your opponent's argument before taking away from it.
  The third rule can be considered a modified version of the principle of charity except this version will get you what you want far more often. As a point in favor of the utility of emotive arguments in the face of an otherwise purely objective and logic-based discussion, Rochefoucauld states, "The passions are the only advocates which always persuade. They are a natural art, the rules of which are infallible; and the simplest man with passion will be more persuasive than the most eloquent without."

Ethics is the study of good and evil as right and wrong, the qualitaties of behavior, and the systematization of values. For this reason, much of traditional ethical theory has become its own discipline, namely political theory. The main branches of contemporary ethics are virtue theory, deontology, consequentialism, and contractualism.
  Ethical concepts I find useful in regular conversation include the is-ought problem, bugmen, and the distinction between philosophy versus sophistry.

For an on-going list of notable philosophers throughout history, Deniz Cem ÖnduyguHis twitter is also interesting if you want to go through his projects there. has made an incredibly useful and interactive tool showcasing philosophers' major arguments and who they dis/agree with. Related, Eric Schwitzgebel has done some forensics on:
 The 233 Most-Cited Works in the Stanford Encyclopedia of Philosophy
 The 295 Most-Cited Contemporary Authors in the Stanford Encyclopedia of Philosophy
 The Continental/Analytic Divide ... in Philosophy: A Quantitative Analysis

If you want an very concise list of reading materials for learning how the world works and what to do about it, this is the shortest list we came up with. These books all give systematic and convincing views on how society works and what your place in it should be. The order matters because it is necessary to dissolve certain ideas about the world before introducing new ones. Starting with the Stoics is standard because they are the most pragmatic and ending with the French sets you up for being able to enjoy the pleasures of life in a way that no other culture does. The list is as follows:
 1. Meditations by Aurelius, 137 pages.
 2. Dokkodo by Musashi, 1 page.
 3. Diogenes Laertius by Plato, ~12 pages.
 4. La Maximes by Rochefoucauld, ~40 pages.

Reading philosophy in historical order is important since most thinkers are directly responding to the generation that came right before them, and not knowing or understanding the perennial issues that underpin their works means you will arrive at a pretty poor interpretation of what you read. So the correct order of modern philosophy is as follows:
  The Enlightenment is what birthed people like Descartes and Thomas Hobbes (or vice versa since people like Descartes are why we call it the Englightenment to begin with) and Descartes and Hobbes are what people like Spinoza, Leibniz (plus Newton on this front), and John Locke are responding to. John Lock is who Berkeley, Thomas Paine, and Rousseau are directly responding to, Berkeley is who Hume is responding to, Hume is who Kant is responding to, Kant is who Fichte and other German Idealists (like Schiller) are responding to, all the German Idealists are who Hegel is responding to, Hegel is who Marx, William James, and Schopenhauer (Schopenhauer is also the first major atheist in modernity - noting this because it marks modern atheism as only 170 years old) are responding to, Schopenhauer is who Husserl and Nietzsche are responding to, Nietzsche, Husserl, and Kierkegaard are who all the Existentialists (Sartre, Beauvoir, Jaspers, Camu, arguably Nishida, etc.) are responding to, Marx and friends are who Weber and Gramsci are responding to (important for Frankfurt school), and Positivists have begun existing here as well (Gottlob Frege, Alfred North Whitehead, Bertrand Russell, G. E. Moore, AJ Ayer, W.V.O. Quine, Wittgenstein, etc.).
  The Existentialists and Positivists are who the rest of the 19th-century philosophers (split into structuralists like Nozick, Popper, Rawls, Merleau-Ponty, Adorno, Horkheimer, and post-structuralists like Deleuze, Guattari, Foucault, Derrida, and Bataille) are responding to, and those are who contemporaries (Zizek, Chomsky, Chalmers, Ray Brassier, Nick Land, Habermas, Sloterdijk, Kripke, Nagel, Dennett, etc.) are responding to. There are a lot of contemporaries focused on older stuff rather than trying to further discussion on new things, like Markus Gabriel who has tasked himself with trying to 'fix' German Idealism, but these people you can mostly ignore.