3. The Stanford Encyclopedia of Philosophy the most comprehensive philosophy reference on Earth, free since 1995. The Stanford Encyclopedia of Philosophy (SEP) is not a Wikipedia-style reference. It's an academic-grade, peer-reviewed, expert-written encyclopedia that covers every topic in philosophy from Plato to artificial intelligence ethics to the philosophy of quantum mechanics. What makes it different: Every entry is written by a recognized expert in the field typically a tenured professor at a major university. Every entry is peer-reviewed by an editorial board of leading philosophers. Every entry is regularly updated to reflect the latest scholarship unlike textbooks that go out of date the year they're published. The scope: 1,800+ entries covering ethics, metaphysics, epistemology, logic, political philosophy, philosophy of mind, philosophy of science, philosophy of language, aesthetics, philosophy of religion, and hundreds of sub-topics. Each entry is 5,000-30,000 words comprehensive enough to replace an entire textbook chapter on the subject. The URL: plato.stanford.edu. Free. No paywall. No account required. No subscription. A philosophy professor told his students: "You used to need $3,000 in textbooks to cover the topics in a 4-year philosophy degree. Now you need one URL. The Stanford Encyclopedia of Philosophy is more comprehensive, more current, and more rigorously maintained than any textbook collection on any shelf. It's been free since 1995. Most of the internet doesn't know it exists because it's not on social media, it doesn't run ads, and it doesn't compete for attention. It just sits there the greatest philosophy resource ever assembled waiting for someone to type the URL."